IMO C
a) n is not divisible by 2. thus (n-1) and (n+1) are divisible by 2.
Also as they are consecutive even numbers, atleast one of them is divisible by 4.
Thus (n-1)(n+1) is divisible by 8
Insufficient. if n=1,r=0 ; if n=3;r=8 insufficient
b)n is not divisible by 3. thus one of (n-1) or (n+1) is divisible by 3.

Thus (n-1)(n+1) is divisible by 3
Insufficient. if n=1,r=0 ; if n=5;r=8 insufficient

a&b together) (n-1)(n+1) is divisible by 24. Thus r= 0 always
Sufficient