While for years several scientists attempted but could not

Once you eliminate the part "but could not", it can be realised that "to" is required after "attempted" because "X attempted prove" is wrong - the correct construction is "X attempted TO prove". Hence we can eliminate A and B.

D is wrong because a subordinate clause (here the "while" clause) must have a verb - "having attempted" is not a verb, but a present participle.

C and E are both constructionally correct, but the word "prove" need not be repeated - because of concision E is better than C.

While for years several scientists attempted but could not prove that so-called Planet X was a tenth planet in our solar system, later scientists convincingly disproved the possibility.

(A) attempted but could not - attempted to is the correct idiom
(B) attempted but were unable- unable to is required + same error as in A
(C) attempted to prove but could not- prove is understood,
(D) having attempted to prove, could not- awkward construction
(E) attempted to but could not - crisp and correct usage of idiom attempted to

KAPLAN OFFICIAL EXPLANATION:

This sentence demonstrates a very common problem by omitting a necessary preposition. Attempted must always be followed by to in order for its use to be idiomatically correct, but the original sentence omits the to. The original sentence, without ellipsis, would read ...several scientists attempted [prove that Planet X...] but could not prove that Planet X.... The necessary to cannot be implied because it doesn't exist in the paralleled phrase could not prove. Therefore, even though the change may seem to make the sentence wordier, the original must be changed to attempted to but could not. Eliminate (A) and (В). (C) is a wordier and clumsier version of (E), so eliminate (C). Finally, (D) breaks the rules of parallelism (among others); having attempted to prove is not parallel with could not prove. (E) wins.

An 800 test taker realizes that a sentence can't imply what it hasn't stated elsewhere. She makes sure that each phrase has everything it needs for its meaning to be clear.

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