Restate the question as variables:
is (xy + z)/(x+1) = 1.5y?
xy + z = 1.5xy + 1.5y
.5xy = z - 1.5y
xy = 2z - 3y
1) No idea what number z is - NS
2) 2z - 3y = xy - Sufficient as per restatment
Another way to think of this is to try some numbers. Clearly A is insufficient we need to know something about z, but with 2), you could try something like the following:
assume x = 1, y = 1, by 2) z must = 2
average before: 1, average after: 3 / 2 = 1.5. Exactly 50% increase.
assume x = 2, y = 4, by 2) z = 10
average before: 8 / 2 = 4, average after 18 / 3 = 6. Exactly 50% increase.
Seems safe to go with B.