Final answer:
A town's population that grows at a constant percentage rate can be modeled by an exponential function, which is exemplified in option C.
Step-by-step explanation:
The context that can be modeled by an exponential function is option C: A town’s population grows at a rate of 4.3% every year. This is because exponential growth occurs when the rate of growth, expressed as a percentage, is constant over time. Each year the population grows by a fixed percentage, which compounds over time leading to the characteristic exponential curve.
In contrast, options A, B, and D are not best modeled by an exponential function. Snow falling at a constant rate (A), a fixed service cost with a linear charge per downloaded song (B), and an elevator moving at a constant speed (D) each reflect linear, rather than exponential, characteristics.