The correct option to fill in the blanks in the sentence “Had he known in advance, he …” is C. would have prepared for the interview. So, the completed sentence is: “Had he known in advance, he would have prepared for the interview.”

Here’s why:

• A. should prepare for the interview: This option is incorrect because “should prepare” is used to give advice or suggestions, not to talk about hypothetical past actions.

• B. was going to prepare for the interview: This option is incorrect because “was going to” is used to talk about intentions or plans in the past, not hypothetical past actions.

• C. would have prepared for the interview: This is the correct option. “Would have prepared” is used to talk about hypothetical actions in the past, which fits with the structure of the sentence.

• D. will have been preparing for the interview: This option is incorrect because “will have been preparing” is used to talk about actions that will be ongoing at a specific time in the future, not hypothetical past actions.

• E. may be preparing for the interview: This option is incorrect because “may be preparing” is used to talk about possible current actions, not hypothetical past actions.