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

1. Retention: Decreases 25%, keeps 75% = 0.75 every 6 hours.
2. Periods: $\frac{24}{6} = 4$ periods.
3. Formula: $C(t) = 800(0.75)^{t/6}$ where $t = 24$.
4. Calculate: $C(24) = 800(0.75)^4 = 800(0.31640625) \approx 253.13$.

💡 Strategic Tip: 4 periods of 75% retention = $(0.75)^4 \approx 0.316$.

Choice B is incorrect because this assumes 75% loss, not retention. Choice C is incorrect because this is after 1 period. Choice D is incorrect because the calculation is wrong.