I would say A

With positive integer:

1) Sufficient

If you multiply Y by itself several times and get to a number X, of course, X is a multpile of Y...

E.g.: 3 * 2^2 + 7 * 2 = 12 + 14 = 26 --> 26 is a multiple of 2

(2) Not sufficient

With this one, we can still have both cases
X=3 --> x^2 - x = 9 - 3 = 6
--> Let's say Y = 3 --> OK, the answer (6) is a multiple of Y
--> X is a multiple of Y

--> Let's say Y = 6 --> OK, the answer (6) is a multiple of Y
--> X is NOT a multiple of Y


This would be my answer...

a) 3y^2+7y=x
y(3y+7)=x
since x and y are both integers, (3y+7) is an integer, say N.
we can say, y*N=x
Clearly N being an integer, (N times y)=x
Hence x is a multiple of y
SUFFICIENT

b) x^2-x is a multiple of y.
Or, Mx is a multiple of y. Clearly y is a multiple of x but nothing can be clearly said about x being a multiple of y.
NOT SUFFICIENT


My answer is A

Cheers!
Aks