选B
这是一句定语从句,
这里的which指代grouds
改成比较看得懂的就是
The professor can hardly find grounds which to base his argument on.
可以拆成两句哦
（1）The professor can hardly find grounds.
（2）The professor's(这里的his指代的是教授的) argument is based on grounds.
或 The professor bases his argument on grounds.
总结一下意思就是：教授几乎找不到能把他的理论建立在上面的论据.