An international group of more than 2,000 scientists project an average global warming that will be between 1.8 and 6.3 degrees Fahrenheit by the year 2000.
A. project an average global warming that will be between 1.8 and
B. project an average global warming to be from 1.8 to
C. project global warming that will average between 1.8 and
D. projects global warming to average from 1.8 to
E. projects an average global warming of between 1.8 and

I was able to narrow down to two choices "D" and "E" based on subject("An international group") and verb ("projects") agreement. But Not able to coorectly identify errors and pinpoint the correct answer? Pls can you identyfy which one is correct and what errors wrong answer has?

Is "of between" in (E) a valid construct ? Is this a prepositional phrase?

well, d has two problems, at least:
- 'projects X to do Y' is unidiomatic. (you can just say 'projects NOUN', as is done in choice e, or you can say something like 'projects that X will do Y').
- an average is a single data point, so there's no such thing as 'averag[ing] from 1.8 to 6.3'. on the other hand, it's quite possible for a single data point to fall between two given values.

i've seen 'of between' in this sense before - think of the phrase 'between 1.8 and 6.3' as standing for a single number, and parse the sentence accordingly (it reads as if it said 'average g.w. of 5 degrees', for instance) - so, if the original poster has copied the problem correctly, we now know that the gmat accepts that construction.

Can 'warming' be used as a countable noun? The Oxford dictionary says it's uncountable, while the Longman dictionary says it is 'singular', in which case the usage of 'a warming' can be possible, but sadly, the Longman does give any examples like this.

BTW, is my wording 'in which case' above acceptable in gmat? ;)

I am taking the test tomorrow, so I would really appreciate it if anyone could give a firm answer please. Thanks a lot!

i've never seen 'in which case' in an official problem, so i'd be suspicious of it. i can't think of an acceptable substitute in ten seconds or less, though, so ... maybe.

you can have 'an average global warming of x degrees', in the same way that you can have the following:
i really resented the removal of program x from my hard drive, a removal that caused many of my other programs to malfunction.

in both cases - removal and warming - the singular is absolutely required: you can't say 'removals' or 'warmings'. nevertheless, there are certain contexts in which articles may placed in front of them; these two are examples. you may be well advised just to memorize those examples.