hmm... i like (a) best, but only if it was supposed to say 'it will', and you mis-typed it as 'will it'. if the version you've typed is indeed faithful to the original, then, well, all the choices suck.
reasons:
- 'whether...' and 'what impact...' are parallel.
- the meaning is correct.
- the tenses are parallel: 'will occur' || 'will have'.
choice b:
- the original sentence clearly indicates that scientists don't know whether warming will occur in the first place. this wording, though, assumes that warming will occur; according to this sentence, the only thing in doubt is the extent of such warming.
- 'whether...' is not parallel to 'impact'.
choice c:
- 'as to' is dicey.
- 'or' should be 'and' (because they're wondering about both questions).
- past tense 'did' is inappropriate.
choice d:
- 'or' should be 'and'.
- 'whether...' isn't parallel to 'the impact'.
choice e:
- tense inconsistency: 'will occur' isn't parallel to 'would have'. moreover, 'would have' isn't appropriate for the consequences of something that hasn't even happened yet.
if choice a actually has the original wording posted at the beginning of this thread ('will it' instead of 'it will'), then it's definitely wrong. 'will it' is only ok in the context of a question ('will it rain tomorrow?'), and can't be used as a noun phrase.
process of elimination:
first, i hope it's clear that we want AND, not OR. according to the context of the problem, the scientific community is divided on both of these issues (you don't get a choice between them), so 'and' makes more sense than 'or'.
that leaves choices b and e.
use PARALLELISM to resolve that dilemma:
choice b uses whether... and the impact in parallel.
choice e uses whether... and what impact... in parallel.
thus, choice e has better parallelism.
(incidentally, the same parallelism issue can also be used to get rid of answers c and d, the ones containing 'or')
hope that helps.
we can justify 'would' here by saying that it's a case of the subjunctive mood, which isn't often used in contexts like this one. they're using 'would' instead of 'will' because the occurrence whose consequences are being considered is hypothetical.
BY Ron