Why don't you help instead ______ A just of complaining B just of complain C of just complainingD just to complain

Why don't you help instead ______ A just of complaining B just of complain C of just complaining
D just to complain

选C
instead of 是固定用法,表示替代的含义.
这句话的意思是,为什么你不停止抱怨,来帮忙?（用帮忙代替抱怨.）

C
instead of 固定词组