Topic: Conditional Sentence,

Best Answer Explained in Below-:

The sentence "If I - the answer, I’d tell you" utilizes a conditional structure known as the second conditional. This form indicates a hypothetical situation that is contrary to reality. In this case, the speaker does not know the answer at present, suggesting a scenario that is not true.

To complete the conditional clause, we need the correct verb form. The options presented all convey different tenses and meanings:

- **A) know**: This implies present knowledge, but does not fit the hypothetical past nature of the condition.
- **B) would know**: This sounds awkward because it doesn’t align with the structure of a second conditional statement.
- **C) have known**: This is a perfect tense form and does not indicate the hypothetical past situation required here.
- **D) knew**: This is the simple past tense, which fits the second conditional perfectly. It implies that in a hypothetical situation where the speaker did possess the knowledge in the past, they would be able to inform someone of the answer now.

Thus, "If I knew the answer, I’d tell you" correctly depicts a scenario where the speaker is acknowledging they do not currently know. Consequently, option D is the contextual and grammatically correct choice.