The more he reads,the more questions he will think of---.A.a

是D正确.
...,the more questions he will think of to ask中,he will think of questions 是主动宾结构.
to ask 是不定式作后置定语,修饰quesions,意思是“要问的”
The more he reads,he will think of the more questions to ask.
（他读得越多,他想到要问的问题就越多.）