Answer:
B. b > 1
Explanation:
When the function is ...
f(x) = a·b^x
it is a growth function when b > 1, and a decay function when b < 1.
__
Additional comment
If you have f(x) = a·b^-x with a negative multiplying the exponent, it is equivalent to a·(1/b)^x, which will be a growth function if (1/b) > 1, or b < 1. In short, pay attention to the sign of the exponent as well as the base value.