[单选题]His writing is so confusing that it's difficult to make out（ ）it is he is trying to express.
A
that
B
how
C
who
D
what
正确答案：D
题目解析
本题考查what的用法。
由题干可知what it is he is trying to express做的是make out（理解，明白）的宾语从句，这个宾语从句中，主语是it，is是系动词，what是表语，he is trying to express做的是what的定语，用来修饰what，且what还要充当express的宾语。故D选项符合题意。
