The professor could hardly find sufficient grounds on which to base his arguments in favor of the

theory 此句话怎么翻译呢？ which 前面怎么用on呢

可以这样理解:base sth. on sth.这个词组的意思应该是"把...建立在...基础之上,某事物以另一事物为依据"这个句子可以分解为: The professor could hardly find sufficient grounds to base his argument in favor of the new theory on (sufficient grounds).用 which 指代suffficient grounds 那么介词on 就要放在 which前面.
这位教授很难为支持这个理论找到足够的理由

您好！很高兴为您解答！ 翻译：这位教授很难找到足够的理由和依据的理论来支撑他的论点 be based on the grounds，中be based on 是一个固定的搭配，which是修饰先行词grounds，所以用on which 希望可以帮到您，祝您成功！

选d...which指代ground...
前面的介词on必须有，因为这里相当于用了base on the ground这个短语..在定语从句中当用which指代了ground, ground前面的on还是应该存在.但这时候自然要加在 which前面，所以变成了on which to base