Choice A is the correct answer. Apply the doubling pattern.

1. Time Periods: 6 hours = 3 periods of 2 hours each.
2. Formula: $V(t) = 500(2)^{t/2}$ where $t$ is hours.
3. Calculate: $V(6) = 500(2)^{6/2} = 500(2)^3 = 500(8) = 4000$.

💡 Strategic Tip: Number of doublings = $\frac{\text{total time}}{\text{doubling period}}$.

Choice B is incorrect because this adds 500 three times (linear). Choice C is incorrect because this is $500 \times 6$. Choice D is incorrect because this is only one doubling.