hmmmmm. My reasoning is getting me to choose E as answer. waiting to know the reasoning for correct answer.

As X is a factor of Y and Y factor of Z, x is a factor of Z

Both statements are by themselves sufficient :

A) XZ is even.
For XZ to be even, X-odd Z- even, X- even Z - odd, X- even Z- even.
As X is a factor of Z, X-even and Z- odd is not possible as anything multiplied by even is even. So in remaining both cases Z is even.
B) Y is even.
As Y is factor of Z and Y is even, Z has to be even, again by same logic that even * (even or odd) = even

i solved too quick and picked B. darn it!