—¿cuándo la camisa azul? me gusta.

— en enero.

a. compró

b. compraste

c. cuestan

d. cuesta

(its not c or d)

B. compraste.

Entire sentence: ¿Cuándo compraste la camisa azul? Me gusta.

Translation: When did you buy the blue shirt? I like it.

Explanation: The question is when, so the answer should be when something was done. The only verb that answers that question is to buy. As you are asking a interlocutor, the subject of the question is "tú" or "usted", depending on whether the second standard or formal person is used. As it is not indicated, the most frequent is to use the standard form. ¿Cuándo compraste tú?, but if you want to use the formal second-person, it would be: ¿Cuándo compró usted? The question is ambiguous.