His writing is so confusing that it's difficult to make out it is he is trying to express

问题补充：

His writing is so confusing that its difficult to make out it is he is trying to express.(·安徽，33)

A. that

B. how

C. who

D. what

答案：

A. that

B. how

C. who

(答案→)D. what

解析：句意：他的写作如此令人费解，很难弄清楚他到底想要说什么。what作express的宾语。