first, the three criteria upon which you're supposed to decide this problem. then, i'll point out one error in the problem (or just an example of bad writing, if you prefer to think of it that way instead) and one stylistic difference from the actual practices of the gmat.

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criterion #1: "difference(s) in" vs. "difference between"
these have different meanings.

"difference(s) in" refers to discrepancies between different measurements of the same quantity. in this problem, that would mean fluctuations in the ocean pressure, or fluctuations in the pressure of the human body.
this is not what is meant.

"difference between" correctly refers to the actual difference, well, between the TWO different things. sorry for using "different" so many times in one sentence.
this is fairly self-explanatory, and it's the correct meaning.

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criterion #2: "he" vs. "they"

the antecedent is "a diver", so you need the singular "he".
see criticism below.

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criterion #3: "ascend" vs. "ascends"

this is prefaced by "it is essential that...", so it must use the command subjunctive. the command subjunctive takes the form of the infinitive (without the initial "to"), so "ascend" is the proper form.

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so the correct answer is (e), a fact that can actually be ascertained from the first two criteria alone.

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2 criticisms:

1, false parallelism - this is actually WRONG in all five choices
in the parallel construction at the beginning of each choice, "the ocean's" or "that of the ocean" must refer to internal pressure.
this doesn't make sense, though, because the ocean doesn't have an internal pressure - it just has a pressure. so you actually can't create a parallel structure here at all, unless you divest yourself of the adjective "internal" (as in the pressure of the ocean vs. that inside the human body).
this may seem trifling, but believe me that it'd be a big deal on the real gmat.

2, the gmat, which is politically correct to a fault, would never dream of using only the masculine pronoun "he".
if this were an official problem, there is no doubt that the singular pronoun would show up as "he or she".

raunekk wrote:thanks lunarpower for that amazing explanation...

The main reason i posted this question is the usage of "verb" in subjunctive mood...

Suppose we take a sentece...

I insist that the chairman resign! (subjunctive mood)

As it is subjuctive mood... it wud be "resign" and not "resigns"...

So does that mean in subjunctive mood.. it doesnt matter if the subject is singular or plural ..the "verb" will remain the same ...

i.e"resign"!!!

thank u..

correct.