Recently Airtel stopped giving the 10+2M offer(i.e. pay for 10 months and get 12 months subscription). Now they replaced it with 11+1M offer(i.e. pay for 11 months and get 12 months subscription).
So in your query the Rs.1850 pack is the 10+2M offer and Rs.2035 is the 11+1M offer.
You could just try for activating the Rs.1850 pack so that you could get 1 more month. If this fails take the Rs.2035 offer.