Final answer:
Option D, f(x) = 3(2)^x, represents an exponential function.
Step-by-step explanation:
The equation that represents an exponential function is f(x) = 3(2)x. The exponential function has the form y = ab^x, where a and b are constants, and b is greater than 0 and not equal to 1. In this equation, b is equal to 2, which satisfies the condition. Therefore, option D, f(x) = 3(2)x, represents an exponential function.