Correct Answer:
A. have/bought
The correct answer is A: have/bought. This sentence requires the Present Perfect tense. The Present Perfect is used to describe an action that happened at an unspecified time in the past, or an action that started in the past and continues to the present, or an action completed in the past with a result that is relevant to the present. The word "already" is a strong indicator for the Present Perfect, signifying that the action of buying tickets has been completed before now.
- B: had/buy is grammatically incorrect; it should be "had bought" for Past Perfect.
- C: has/buy is grammatically incorrect; it should be "has bought" for Present Perfect (if the subject were singular).
- D: did/bought uses Past Simple, which would imply a specific past time, and "already" doesn't fit as naturally.
Therefore, "We have already bought the tickets" correctly conveys a completed action with present relevance.