I'm sure he understands ________ you feel. He's very clever.

I'm sure he understands ________ you feel. He's very clever.
A、when
B、what
C、whether
D、how
【正确答案】：D
【题目解析】：根据四个选项中各个疑问代词#_#疑问副词的意义，结合句意，可知应选D项。