I can't decide which one to buy,to buy 修饰one.这是宾语从句（动词和问句Can you decide which one to buy 一样要用倒装）希望你能理解这种从句句式写法,那为什么a不可以？一定要用宾语从句吗？是的，英语句型最好写成 which one to buy，这题选B最合适