从 can you 可以看出这是一个倒装句,所以只有B.Only on;whomever 正确了.
由 Only + 名词/代词/状语从句等 构成的短语放在句首,需倒装.
如：
Only in these way can you improve your spoken English.
Only when she knew the answer did she realize that she had made a mistake.