Let's break it down into two parts:
(4/3) + (7n/3)
Statement 1 tells us, that n+1 is divisible by 3, which means n must be 2,5,8,11.......
The remainder when 4 divided by 3 is 1 and when we replace the value of n in (7n/3) then we will have 2 as the remainder.
Add both remainders 1 + 2 which will be 3.
and 3/3 is quotient 1 and remainder 0. -> Sufficient.
Statement 2 tells us n > 20.
(4 + 7*21)/3 = 151/3 -> Remainder 1.
(4 + 7*22)/3 = 158/3 -> Remainder 2.
Insufficient
So Answer is A