The professor could hardil find sufficient grounds _____hia arguments in favor of the new theory.

这是 base A on B 句型 以乙作 甲的基础 base his arguments on sufficient grounds 以足够大理由作辩论 的根据
如果选B 的 话 应该是The professor could hardly find sufficient grounds to base his arguments on in favor of the new theory.
这是 动词不定式 作定语 与所修饰词 是动宾关系
但是 本句 on的位置不对
所以 答案是 D 其实也是不定式作定语 只不过 前面加了on which 相当于一个定语从句
=on which he based his argumentsA选项有be based on这种用法吗因为不定式to base 的逻辑主语 是句子主语 所以 应该用主动形式 作定语 与所修饰词 是动宾关系
也就是 The professor based his arguments on sufficientgrounds

所以 to be based on作定语 不对 应该是 to basehis arguments on