IMO the correct answer is D
Unlike football, which is played on standardized fields,
baseball fields vary considerably in both size and shape.A.
baseball fields vary considerably in both size and shape - "Unlike football, ...." - "football" is incorrectly compared with "baseball fields" - Incorrect
B. baseball fields vary considerably, in size as well as in shape - Same as option A
C. baseball is played on a field that varies considerably in both size and shape - The above error is rectified, whereas, a new meaning error is introduced...."on a field that varies considerably in both size and shape" - meaning a single field can vary in size and shape - Incorrect
D. baseball is played on fields that vary considerably in both size and shape - CORRECT
E. baseball is played on a field that varies considerably in size as well as in shape - Same as Option C