Research during the past several decades on the nature of language and the processes that produce and make it understandable has revealed great complexity instead of underlying simplicity.

(a)that produce and make it understandable has revealed great complexity instead of underlying simplicity.

(b)that produce and make it understandable have revealed great complexity instead of underlying simplicity

(c)by which it is produced and understood has revealed not underlying simplicity but great complexity

(d)by which it is produced and understood have revealed great complexity rather than underlying simplicity

(e)by which one produces and understands it have revealed great complexity instead of underlying simplicity

答案选(a)我觉得这明显不对呀，这句话的主语应该是research和precesses吧，谓语动词应该是have呀，怎么会是has呢？

我选了d，不知道对不对。。。

句子的主语是Research，后面一大串都是做research的后置定语，说明research的内容
句子的主干是Research has revealed great complexity instead of underlying simplicity，所以谓语动词肯定是单数

D用了被动语态，显得很繁琐~awkward and wordy