Choice B is the correct answer. The decay factor is the base of the exponent.

1. Identify: In the form $y = ab^x$, the base $b$ represents the growth or decay factor.
2. Locate: In $P(t) = 500(0.85)^t$, the base is 0.85.
3. Interpret: Since $0 < 0.85 < 1$, it represents decay, and 0.85 is the decay factor.

💡 Strategic Tip: The factor $b$ is what you multiply by each step. If $b < 1$, the quantity decreases.

Choice A is incorrect because 500 is the initial population. Choice C is incorrect because 0.15 represents the decay rate ($1 - 0.85 = 0.15$ or 15%), not the factor. Choice D is incorrect because 15 is the percentage rate, not the factor.