______ excites the artist is that he has been allowed to hold an exhibition.

分析句子结构可知，is之前是一个主语从句。从句中缺少主语，因此需要使用一个疑问词来引导这个主语从句，表示“什么”的事物。选项A的it不能引导从句；选项B的when在名词性从句中作时间状语；选项D的which在名词性从句中需要有选择含义，不符合题意。因此，正确答案是C，即What excites the artist is that he has been allowed to hold an exhibition。