Final answer:
Option D, f(x) = 2^x, is the exponential parent function as it fits the standard form f(x) = b^x without additional terms or modifications.
Step-by-step explanation:
The student asked which of the given functions is an exponential parent function. The exponential parent function is a function of the form f(x) = bx where b is a constant, and x is the exponent. By looking at the provided options, we notice that only option D. f(x) = 2x fits the description of an exponential parent function. Unlike the other choices, choice D doesn't have additional terms or modifications to the basic form f(x) = bx.