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 -> group is singular, so we need singular verb. Incorrect.
(B) project an average global warming to be from 1.8 to -> Same as A. Incorrect.
(C) project global warming that will average between 1.8 and -> Same as A. Incorrect.
(D) projects global warming to average from 1.8 to -> average global warming makes sense. Incorrect.
(E) projects an average global warming of between 1.8 and -> It is better. We have subject verb agreement issue resolved and "average" is at right place.
So, I think E.