c it is. good question with very good options.

Let me explain the thought process for this one (since only 36% have got it correct so far)
This question actually goes back to the basics ( correlation , causation etc. )

Firstly, since it's an assumption question, we must find an unstated premise which when added to the argument will logically prove the conclusion. So we need to find an option which tells us that the effect of global warming on ice volume should be opposite of what is presented over here.
Option (a) & (d) talk about correlation, but provide no conclusive cause-effect relationship =>> Hence, eliminated.
option (e) is necessary but not sufficient (again no conclusive evidence) ----->> Eliminated
option (b) states a valid point but it only seeks to establish that the cold winds caused the increase in ice volume. Nothing is explicitly stated about global warming -----> Eliminated

option (c) is the only one which gives us what we were looking for. Since global warming CAUSES melting of ice, so.........

Although the options are seemingly quite close, but still there is ONLY ONE correct answer. The rest are flawed.
That's GMAT

2 situations can lead the Global Warming to be hoax:

i) Global warming was expected to be the reason behind the sea ice level but "the strong westerly winds in the South Pole" took all the credit.
ii) Global warming was expected to counter the effect of the [color=#005e20]the strong westerly winds in the South Pole but nothing happened.[/color]

Note: i) leads to choice D and ii) to choice C.
But, D does not clearly state that Global Warming is the reason behind sea ice level, but claims a greater correlation. Greater to what?? Unclear.
C directly talks in terms of (ii).

So, C wins.