i just wonder _ that makes him so excited.a\why it does b\what he does c\how it is d\what it is

首先，“使他如此兴奋”一定是一件事或物所以只可以是what。
其次，空格后面的部分是wonder的宾语从句。如果是b，宾语从句就成为：
what he does that makes him so excited.他所做的事就是what he does很明显多了that。而d，宾语从句成为：
what it is that makes him so excited.是What makes him so excited的强调主语的形式。