Hence option A is the correct answer.

Hence option B is the correct answer.

Let’s see other options-

In option A , since is used which is used to provide a reason.

In option B, assume is used and it doesn’t give any sense.

“Let’s it starts with sanitation” will be replaced with “and starts with sanitation”.

Hence, option C is the correct answer.

Hence, option B is the correct answer.

The underlined part is grammatically incorrect. ‘Of’ preposition should be replaced with ‘since’ which means ‘from a particular time in the past until a later time, or until now’. As present perfect tense (has/ have + v3) is used here, ‘since’ is the most suitable choice.

Hence, option B is the correct answer.

The underlined part is grammatically incorrect. There is no such word as ‘gainings’. It should be ‘gaining’ a noun which means have the benefit of; obtain something desirable. For noun ‘mothers’ pronoun ‘their’ should be used. And preposition ‘from’ should be used before ‘hearing’ as mothers gain happiness from hearing their children’s smile. The most suited one is “gaining their happiness from hearing their”.

Hence, option A is the correct answer.

The underlined part is grammatically incorrect. The plural noun ‘teachers’ needs a plural verb i.e. ‘require’. For noun ‘teachers’, pronoun ‘us’ should be used. Article ‘a’ should be used before adjective ‘straight’ i.e. we walk in a straight line of conformity. Thus, the most suited one is “require us to walk in a straight line of”.

Hence, option A is the correct answer.

The underlined part is grammatically incorrect. For the plural noun 'Documentary makers', we need a plural verb or verb’s second form. There is no plural verb in the underlined part and in the options given; thus ‘highlighted’ should be used. As there is no comparison being made in the sentence, the usage of adjective ‘strong’ is correct. In the sentence, slavery and today's prison system are being discussed (which are two in number). Therefore, we will use 'between' in place of 'among'. Thus, the suitable option is “highlighted the strong similarity between”.

Hence, option B is the correct answer.

The underlined part is grammatically incorrect. With preposition ‘to’, we always use a verb’s first form i.e. ‘help’. It should be ‘people’ in place of 'peoples'. And preposition ‘in’ should be used before the noun ‘city’ as it is used to express the situation of something that is or appears to be enclosed or surrounded by something else. The most suited one is “to help the impecunious people in the”.

Hence, option C is the correct answer.

 The widely used phrase is not ‘becomes the key’ but ‘is the key’ which means that something is the only, or most important, factor. Thus the underlined part needs an improvement given in option C. Hence, option C is the correct answer.