Choice C is the correct answer. Convert percentage decrease to a decay factor.

1. Identify Rate: Decrease of 20% means $r = 0.20$.
2. Calculate Factor: $b = 1 - r = 1 - 0.20 = 0.80$.
3. Form Equation: $y = 500(0.80)^t$.

💡 Strategic Tip: If you lose 20%, you keep 80%. The factor is what you keep.

Choice A is incorrect because 0.20 would mean losing 80% per year. Choice B is incorrect because 1.20 represents a 20% increase. Choice D is incorrect because it has the wrong structure entirely.