Which of the following options results in a graph that shows exponential decay?
f(x) = 0.6(2)x
f(x) = 3(0.7)x
f(x) = 0.4(1.6)x
f(x) = 20(3)x

f(x) = 3(0.7)^x

Because as the exponent (x) of 0.7 grows the result decreases. When x is a big negative number the decay is fast, when the x is a big positive number the decay is low, and the function appraches assymptotically to a constant value. This is the behavior of an exponentially decaying function.