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).

Whereas heat transferred between the ocean and the atmosphere occurs more rapidly when the ocean is warmer than the air, it is slow when the ocean is colder.
(A) Whereas heat transferred between the ocean and the atmosphere occurs more rapidly when the ocean is warmer than the air, it is slow when the ocean is colder.
(B) Unlike the slow process of heat transfer between the ocean and the atmosphere when the ocean is colder than the air, heat transfers more rapidly when the ocean is warmer.
(C) The transfer of heat between the ocean and the atmosphere is slow when the ocean is colder than the air and more rapid when the ocean is warmer.
(D) Transferring heat between the ocean and the atmosphere occurs slowly when the ocean is colder than the air and more rapidly when it is warmer.
(E) When the ocean is colder than the atmosphere, heat transferred between them occurs slowly, unlike the more rapid transfer occurring when the ocean is warmer.

A and E say 'heat transferred occurs' ... this is plain wrong as heat transferred = heat ... so the sentence can't read 'heat occurs / happens'. It should be 'heat transfer occurs'. Also, in A 'it' should refer to 'heat transfer'. WRONG

B compares 'slow process' to 'heat transfers' ... wrong.

D uses 'transferring heat', which is a gerund (-ING form of the verb used as a noun) form ... usually not preferred if the proper noun form (transfer) is available. Also, 'transferring heat' gives the impression of an 'intended' activity. Wrong!

C: correct.