The opposite of pertinent is NOT impertinent, but irrelevant. Impertinent means "rude," while irrelevant means "unrelated." Pertinent is slightly more descriptive than relevant, as pertinent refers to immediate, precise, and direct relevance.

Pertinent

Definition: Relevant or applicable to a particular matter; on-topic; directly related to the subject at hand.

Explanation:
The correct answer to this question is option 'D', which is "Irrelevant". Let's understand why the other options are incorrect and why 'Irrelevant' is the correct answer.

- a) Irrational: This word refers to something that is not based on reason or logic. It is unrelated to the meaning of 'pertinent'. Therefore, option 'a' is incorrect.

- b) Irregular: This word describes something that is not regular or consistent. It does not convey the meaning of 'pertinent'. Hence, option 'b' is incorrect.

- c) Insistent: This word means being persistent or demanding. Although it shares some similarity with 'pertinent' in terms of intensity, it does not convey the same meaning. Therefore, option 'c' is incorrect.

- d) Irrelevant: This word means not related or not applicable to the matter at hand. It is the correct answer because it is the opposite of 'pertinent'. When something is pertinent, it is directly related and relevant, whereas when something is irrelevant, it is not related and not applicable. Therefore, option 'd' is the correct choice.

In summary, 'pertinent' refers to something that is relevant and directly related to the subject. 'Irrelevant' is the correct answer because it means not related or not applicable, which is the opposite of 'pertinent'. Options 'a', 'b', and 'c' are incorrect because they do not convey the meaning of 'pertinent' accurately.