In my opinion, the answer could be either option (A), option (B) or option (D). Both option (A) and option (D) imply that the 'little sister' bought a laptop for herself on her birthday, which is a normal thing to do. Option (B) implies that someone else had bought a laptop for her, as a gift. Thus, all three options are correct. Option (B) is given as the answer based on the stereotype that a 'little sister' is young, which is inaccurate as she can also be old and able to earn her own money, and buy herself gifts.