谁会这道题可以给我讲讲，为什么用what？从句为什么不完整?

Question

I am beginning to think that————people say about him is true.
A.which B.this C.what D.\

Answer 1

这是一个that引导的宾语从句，宾语从句是个主系表结构，what people say about him是个名词从句，充当宾语从句的主语。