Choice C is the correct answer. Depreciation means decay.

1. Identify Rate: $r = 15\% = 0.15$.
2. Calculate Factor: $b = 1 - r = 1 - 0.15 = 0.85$.
3. Form Equation: $V(t) = 20000(0.85)^t$.

💡 Strategic Tip: For depreciation, the base must be less than 1.

Choice A is incorrect because 0.15 would mean losing 85% per year. Choice B is incorrect because 1.15 means the car gains value. Choice D is incorrect because it represents linear depreciation.