let's assume that you have eliminated A, B, D, because both statements individually are insufficient. (if you need help reaching that conclusion as well, please post and say so, but the fact that you are considering C vs E indicates that you've already gotten that far.)

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let's take the two statements together. perhaps the easiest way to do this is to look for patterns in the data.

the numbers satisfying statement (1) are
14, 28, 42, 56, 70, 84, 98, 112, 126, 140, ...

of these, the ones satisfying statement (2) are boldface:
14, 28, 42, 56, 70, 84, 98, 112, 126, 140, ...

you can see the pattern: every third number is boldface, and these are exactly the numbers that are multiples of 42 (and therefore also multiples of 21).

sufficient
answer = c

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here's the theory-based solution.

statement (1) says that n is equal to 14x, for some whole number x.
statement (2) says that n - 12 is divisible by 3.

taking the two statements together, we have that
14n - 12 is divisible by 3
which means that 14n itself is divisible by 3 (because 12 is divisible by 3)
which means that n is divisible by 3, because there are no 3's in the prime factorization of 14.

sufficient
answer = c