The error is in the second part of the sentence. We use singular verb after singular noun. Here the subject ‘radical change’ is singular. Hence, in place of ‘are’, ‘is’ must be used.

Hence, option B is correct.

The error is in the second part. The use of the adjective ‘worse’ is incorrect and we need the verb ‘worsen’ which means to make or become worse.

Hence, option B is correct.

The error is in the second part. Both ‘many’ and ‘many a’ convey the same meaning that is "a large number of". The only difference is that ‘many’ is used with countable plural nouns followed by plural verb while ‘many a’ is followed by a singular countable noun and takes a singular verb with it. So, in place of ‘look’, ‘looks’ should be used in part two.

Hence, option B is correct.

The error is in the first part of the sentence. Proper verb is not used here. ‘Insure’ means ‘arrange for compensation in the event of damage to or loss of (property), or injury to or the death of (someone), in exchange for regular payments to a company or to the state’. It is wrong in this context and should be replaced by the verb ‘ensure’. It means ‘make certain of obtaining or providing (something)’.

Hence, option A is correct.

The error is in the first part of the sentence. Note that, when “did not” is used in a sentence, the verb must be in its base form. So, the verb “realized” should be replaced by “realize” in the first part.

Hence, option A is correct.

The error is in the third part of the sentence. The sentence is of passive voice, so, third form of verb should be used in the sentence. So, in the third part of the sentence ‘chosen’ should be used instead of ‘chose’.

Hence, option C is correct.

The error is in the first part of the sentence. Note that, we should not use an auxiliary verb and a modal verb together in a sentence. In order to make the sentence correct, remove “are” from the first part of the sentence. ‘Ought’ is ‘used to indicate something that is probable’.

Hence, option A is correct.

The error is in the third part of the sentence. We use had better plus the bare infinitive to give strong advice about the present or future. When had is used to refer to present and future events, it is sometimes called the unreal past. So, preposition “to” should be removed from the third part of the sentence.

Hence, option C is correct.

The error is in the third part of the sentence. When “but” is used as a preposition in a sentence then it is followed by a bare infinitive. So “to” must be removed from the third part to make the sentence correct.

Hence, option C is correct.

The error is in the fourth part of the sentence. “Need” and “dare” are two verbs which can act as modal verbs as well as auxiliary verbs. When we use “not” with them, they act as modal verbs. We cannot use preposition “to” with them. So, “to” should be removed from the fourth part.

Hence, option D is correct.