He is very grateful to the girl he often turns for help when he meets with difficulties.A.

问题补充：

He is very grateful to the girl he often turns for help when he meets with difficulties.

A. whom

B. to whom

C. to who

D. which

答案：

A. whom

(答案→)B. to whom

C. to who

D. which

解析：考查定语从句：先行词是the girl，定语从句中turn后面少to和宾语，turn to求助于，介词后面只能接which或whom，不能接that或who，用to whom引导定语从句，句意：他对那个遇到困难时可以求助的女孩很感激。选B。