'My sister was bought a laptop' is a passive construction, similar to 'My sister was given a present for her birthday'. It means that someone else bought the laptop as a present and gave it to her. Active: (for example) 'My parents bought my sister a laptop for her birthday' or 'My parents bought a laptop for my sister's birthday.' Passive: My sister was bought a laptop for her birthday However, it's a poorly written exam question, because 'had bought' is possible. It's less usual for people to buy themselves presents for their birthdays, but grammatically it's OK.

"Was" as an answer makes no sense to me. Your answer of "had" is the only one that sounds correct to me. "My sister was buying a laptop last year" makes sense. But I had to change a word.