We regret not booking the seats in advance.

The correct answer is: EASY

Explanation: I regret not making a reservation.

Mean:

Actually did not book in advance. Now wish it was booked.

=> This is wishing for something in the past. If you use wish, it must be S + wish + S + had + Ved.

From this it follows that A and B are false.

If using if only, it must be: If only + S + had + Ved.

From this, it follows that C is structurally incorrect.

So the correct answer is D.

Translation: I regret not making a reservation.