Explanation:

Correct answer is (b). In the statements given above, the first states the reason for the growing internet phenomenon and the second points towards the requisites to serve the market.
Applying the elimination technique, let’s check the relevance of each of the options one by one.
Possibility 1.
Even if more than 90 per cent of almost 350 million Indians currently accessing the Internet on mobile devices, the availability, affordability and quality, in terms of browsing speeds, of the data service becomes critical. The formation above, though, looks grammatically correct but doesn’t make any sense contextually. The phrase ‘Even if’ is used for emphasizing that a particular situation will remain the same no matter what happens.
Ex. I’m determined to prove my innocence even if that means going to the highest court in the land.
Ex. I’ll go even if you forbid me to. But, in the compound sentence made above there is no context of emphasizing anything. This hence eliminates option C.
Possibility 2.
Since the availability, affordability and quality, in terms of browsing speeds, of the data service becomes critical, more than 90 per cent of almost 350 million Indians currently accessing the Internet on mobile devices. Once again we have got a sentence which seems correct grammatically, but is absolutely absurd while deriving a meaningful context out of it. We use ‘since’ as a conjunction to introduce a reason and had the word ‘since’ joined with the statement I instead of the statement II, it would have made a meaningful synthesis. The options A and D get eliminated as well.
Possibility 3.
With more than 90 per cent of almost 350 million Indians currently accessing the Internet on mobile devices, the availability, affordability and quality, in terms of browsing speeds, of the data service becomes critical. Clearly, the above synthesis is both grammatically and contextually correct and keeps the flow intact. Option B is hence the correct answer.