Which is an exponential decay function?

An exponential function represents decay when its effective base is between 0 and 1. In choice C, (8/7) raised to the power −x is equivalent to (7/8)^x, and 7/8 is between 0 and 1. Therefore, choice C, f(x) = (3/2)(8/7)^(−x), is an exponential decay function.

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