I encountered into not so difficult DS problem. I guess that VStudy answersheet gives wrong answer. (Book 1, Test 1)
A is not sufficient. For XYZ to be > 0, at least 2 variables has to be -ve or all 3 variables to be +ve.
B is also not sufficient. Both Y & Z can be -ve or +ve ( YZ must have same sign ) to get YZ> 0.
A & B together, let's look at condition with YZ> 0,
X Y Z YZ XYZ X(Y+Z)
1 2 3 6 6 6
1 -2 -3 6 6 -5
As you can see, X(Y+Z) can be +ve or -ve.
Thus, C is still not sufficient. The answer has to be E.