试题题干
His writing is so confusing that it's difficult to make out（ ）it is he is trying to express.
参考答案
正确答案：
试题解析
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。
句意：他写的东西很混乱，所以很难理解他在表达的东西是什么。