he is only student _____can work out the difficult problem in mathsA / B whom C which D that为什么?

he is only student _____can work out the difficult problem in mathsA / B whom C which D that为什么?
he is only student _____can work out the difficult problem in maths
A / B whom C which D that
为什么?

he is only student _____can work out the difficult problem in mathsA / B whom C which D that为什么?
D
他是唯一一个能解出这道数学题的学生.
这是一个定语从句.从句中缺少主语,首先排除A,因为在从句中做主语的关系词不能被省略,再排除B,因为这是宾格,不能做主语,因为先行词student是人,不能用which,which只能指代物.that既能指人,也能指物.

A /
先行词是人，引导词在从句中充当主语，所以只能用who
定语从句中引导词可省略