The underlined part is grammatically incorrect. 

• Let's first see how these relatives pronouns are used:
  
  Who = We use who in relative clauses to refer to people, and sometimes to pet animals.
  e.g. She’s going out with a bloke who is in the army.
  
  Whom = We use whom in formal styles or in writing to refer to people when the person is the object of the verb.
  e.g. The response of those managers whom I have consulted has been very positive. 
  
  Which = We use which in relative clauses to refer to animals and to things. 
  e.g. You need to tick the box which says yes.
  
  
• In the given sentence, the noun for which we will use the pronoun is 'armed forces' (and is subject).
  Thus, we will use 'who'. 
  
  
• Since the relative pronoun 'who' has been used for the plural noun 'armed forces', we will use the plural verb 'are' with it according to the subject-verb agreement rule. 
  
  Subject-verb agreement = The person and number of the subject of the clause determine the person and number of the verb of the clause. A singular subject will take a singular verb and a plural subject will take a plural verb. 
  
  e.g. He is playing. 
  
  e.g. They are playing. 
  
  
• For the noun 'commitment', we need an adjective to describe it. 
  
  "Heavily" is an adverb. 
  
  Thus, replace the adverb 'heavily' with the adjective 'heavy'. 
  
  The correct sentence will be: India has about 1.5 million personnel in its armed forces who are already under heavy commitment in our western and eastern fronts.

Hence, option C is the correct answer.

* We use the second form of a verb when we are talking about activities of simple past.

e.g. He went to the park.

* The first clause is in the simple past tense (longed), so the clause after ‘but’ should be in the past too. So, ‘chased’ in place of ‘chase’ will be used.

* The phrase ‘chase someone away’ means to force someone or something to run away.

* The pronoun ‘her’ will be used for the noun ‘Sivgami’ as it is a girl's name.

The correct sentence would be: Sivgami longed to touch the chisel, but her grandfather chased her away.

Hence, option B is the correct answer.

The underlined part is grammatically incorrect. The first part is in the simple past, while the underlined part has used the simple present tense. Thus, it is incorrect.

• Since the first segment of the sentence mentions the action in the simple past tense (v2), the use of ‘do not pursue’ is incorrect to be used in the second segment.

• The two parts are joined by the conjunction ‘but’ to show the contrast.

e.g. He was called, but he did not answer.

• Simple past tense (v2) would be more appropriate to be used. Thus, ‘did not pursue’ is the suitable replacement of the underlined text.

Hence, option A is the correct answer.

• 'To hear' is an active form of the infinitive.
  
  
• Here, we need a passive form of the infinitive, i.e. 'to be heard' as the person is receiving the action. 
  
  
• Thus, replace the underlined part with the one given in option D.

Hence, option D is the correct answer.  

The underlined part is grammatically incorrect.

• The phrasal verb ‘Enroll at’ means to put yourself or someone else onto the official list of members of a course, college, or group. Thus, the use of ‘at’ is correct.

• The sentence is of past tense. The process of enrollment has already taken place. Thus, we will use the V2 form of the verb ‘enrolled’ in place of ‘enrolling’.

Hence, option C is the correct answer.

The sentence implies that among all the novelists, the subject's favourite was Charles Dickens.

You use 'of all' in expressions such as of all people or of all things when you want to emphasize someone or something surprising.

For example, Jordan, of all people, committed a flagrant foul.

Hence, the underlined segment should be replaced with 'my favourite novelist of all' to make the sentence grammatically correct. Therefore, option B is the correct answer.

The preposition 'in' is used to express the situation of something that is or appears to be enclosed or surrounded by something else.

The preposition 'between' is used to refer to one-to-one relationships.

The preposition 'at' is used to express location or arrival in a particular place or position.

The verb which follows 'make sure' is expressed as an imperative.

For example, make sure you score a goal!', is approximately the same thing as saying 'Score a goal!'.

Also, the phrase 'make sure' means 'to ensure that something is done or happens'.

Moreover, we use 'to + infinitive' in imperative sentences.

Hence, the underlined segment should be replaced with 'make sure to take' to make the sentence grammatically correct.

Hence, option C is the correct answer.

We use participle after 'be' form of the verb. Also, the sentence is in the passive voice. Hence, passive voice of present tense should be used.

Therefore, the underlined segment should be replaced with 'is broken down' to make the sentence grammatically correct. Thus, option A is the correct answer.

Had it been 'breaks down', option B would be correct because the subject-verb agreement would have been maintained in the sentence.

We use the past continuous tense to describe actions or events in a time before now, which began in the past and were still going on when another event occurred.

For example, they were waiting for the bus when the accident happened.

Hence, the underlined segment should be replaced with 'was eagerly waiting' to make the sentence grammatically correct.

Therefore, option D is the correct answer.