GMAT-Prep SC: Final solution at one place:

Important: The purpose of this post (and all the other posts by me) is to give a complete solution to all GMAT-Prep Verbal questions at one place. Sometimes students have to wade through dozens of posts to get to the final answer. My posts will give one complete and crisp solution required to arrive at the correct answer by eliminating the wrong one. Some of the content in these posts may have been taken from various other sources (discussion forums).

Studies that have compared the different types of insects in coffee plantations and tropical forests have reported either finding a similar or a greater variety in plantations.
(A) and tropical forests have reported either finding a similar or a
(B) with those of tropical forests have either reported finding a similar or
(C) with those in tropical forests have reported finding either a similar or a
(D) to tropical forests have either reported finding a similar or
(E) to those of tropical forests have reported either finding a similar or

The only answer that maintains proper 'either X ... or Y' X || Y parallelism is C - 'either a similar or a greater'. All the other options are clearly not parallel.

C: Correct