M.Sc

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Well, it affects during placements. Some companies (very few) would specifically ask for candidates who had no history of arrears. But, many would just allow candidate with one backlog but has cleared it at present. Thus, it is always best to clear it, by third year and get no backlogs in last two years. You might lose few companies. But, if you have already had one, then clear it and move on. Don't lose any hope. But once you pass out and are no longer a fresher, it won't affect much. However, study as if you want no backlogs, who knows you might lose your dream company because of one mistake you made.

Yes, you can do it without any doubt. It will be easier for you as you already a computer Science graduate.
NIT (s) ask for maths as a subject in your 10+2 or graduation level. Rest, there are many universities which doesn't even consider Maths as a criteria.

Hello Deepti,Pant Nagar University is a great choice. For all the details you can visit https://www.google.co.in/amp/s/www.shiksha.com/university/g-b-pant-university-of-agriculture-and-technology-gbpuat/exams/amp/pantnagar-university-entrance-exam. I hope you find everything out and for further details I'm still here to help you anyways. Thanks.

Yes, you are eligible for M.Sc Statistics, but you need to clear an entrance which has 60% statistics and 40% Maths. Prepare for it.