An exponential growth function is of the form f(x) = ab^x, where b > 1. In this case, we have f(x) = Pa^x. To ensure that this is an exponential growth function, we need a to be greater than 1.
Out of the given options, only option D has a value of a that is greater than 1. Therefore, the answer is:
D. P= 9; a = 1/4