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Dear Sukant2010
I'm happy to help.

First, consider the choices on their out, apart from the parallelism in the second half of the sentence:
A. an increasing in numbers = very awkward to use a complex gerund when a simple noun is available.
B. an increase in their numbers = acceptable
C. their increase in numbers = this would pass in colloquial speech, but it is logically suspect. It makes sense to talk about "their numbers", which is equivalent to "the number of them." But does the "increase" belong to them? Something is very murky about this, and the GMAT would avoid it like the plague.
D. increasing in their numbers = awkward to use a gerund when a simple noun is available.
E. an increasing in the numbers of them= very awkward to use a complex gerund when a simple noun is available.

Choice (B) appears to be the only valid choice, just considered on its own. Fortunately, it also works in the parallelism with "a strengthening of their support." When we put an article or a noun-modifier on a gerund, we make it what MGMAT calls a complex gerund. It is perfectly acceptable to have a complex gerund in parallel with an ordinary bonafide noun. Thus, (B) is perfectly correct on all count.

Here, for free, is another GMAT SC practice question:
https://gmat.magoosh.com/questions/3597

Mike