The expression below shows the population of butterflies in a forest after 4x years.

f(x) = 40(1.04)4x

Which of the following is an equivalent function?
a. f(x) = 40(4.1600)x
b. f(x) = 480(1.04)x
c. f(x) = 40(1.1699)x
d. f(x) = 480(4.1600)x