<span>"That these little creatures survive in these small pools of water" is the whole noun clause You could replace that whole noun clause with "That fact" which is a noun phrase and have a sentence That fact is amazing where "that fact" would be the subject.
Likewise, </span>this whole noun clause represents A SUBJECT of the sentence because it corresponds with the predicate "is".
I think it is present perfect.
A is the correct answer because a prepositional phrase is telling where something is located. so a is saying that the envelope is in the drawer.
