OA E

Explanations are welcome..

A. lay in the fact that he proposed the idea
B. lay in the fact of his proposing the idea
C. laid in the fact of his proposing the idea
D. laid in his proposal
E. lay in his proposal

Lay is a special verb in this context. You may want to look up this verb if you chose C or D. B and C are wrong because of a verb tense error with "proposing." Darwin proposed the ideas a long time ago and so the verb must be in the past tense. E is better than A because A is needlessly wordy:

"in the fact that he proposed the idea" can be replaced with "in his proposal" since "his proposal" suggests that he proposed an idea. "In the fact" is not needed.

why not "laid" D?