Sentence Correction

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patanjali.purpose wrote::
1) What does THE FIGURE refer to - "198,113 students" or '42% of students'?

the default for any modifier set off by commas -- unless the context very strongly and clearly suggests otherwise -- is to pertain to the nearest possible referent, i.e., the stuff on the other side of the comma.
so, this particular modifier would be taken to refer to "42% of students...".

i understand why this modifier is a little bit annoying -- specifically, it would also make sense if applied to "198,113 students" -- but the general trend, in interpreting modifiers such as this one, is to assign them to the nearest possible stuff that they can modify.
more importantly, this problem has a clear official precedent: it is closely based -- perhaps a little too closely based -- on #4 from the 11th edition of the official guide.

2) "for 1981" - is this parallel with 'IN 2003'? Why FOR 1981 is correct? Would C be still correct, if C were 'double the figure IN 1981'

this is not a parallel structure. one part is describing an actual state of affairs that prevailed at the mentioned time, while the other is just a statistic mentioned in retrospect.

the reason you don't need "in 1981" is that there is no mention of an explicit event that actually occurred in 1981. that is, if the sentence actually mentioned a past-tense event that occurred in that year, then you would use "in", as in we had our first child in 1981.
on the other hand, "the figure for 1981" is a statistic that continues to exist even after the actual end of the calendar year 1981. this statistic is, in a very real sense, permanent.
if you used "the figure in 1981", you would be suggesting that historical value of some figure that later changed. for instance, the current exchange rate between country x and country y is 9 to 1; the figure in 1981 was 11 to 1.

there is also, of course, an element of idiom here. there's no particularly good reason why it should be "for" and not, say, "of".

3) is D wrong bcoz of ambiguous IT only or there is some other reason as well?

the pronoun is not just "ambiguous", it's actually wrong. there's no noun for which it can stand.