consider 1st option:
The product of abc is divisible by 3, but only c is divisible by 21.
The product of abc is divisible by 3 => atleast one of the a,b,c is a multiple of 3.
but only C is divisible by 21 => c is a multiple of 3 and 7.
for a and b not to be divisible by 21 (7*3) they should not be a multiple of 7 or 3 or both.
there is no information whether a and b is a multiple of 7 or not.
a = 7, b = 3
a = 7, b = 7
a = 10, b = 8
all these satisfy the above conditions.
so we can`t answer with A alone.

consider 2nd option:
Each of the three positive integers is divisible by 3, but only c is divisible by 21.
a = 3k
b = 3m
c = 21n
for a and b not to be divisible by 21 (7*3) they should not be a multiple of 7.
so only C is a multiple of 7.
hence, it can be answered by B alone