Choice A is the correct answer. Calculate the ratio per time period.

1. Period 1: $\frac{800}{100} = 8$.
2. Period 2: $\frac{6400}{800} = 8$.
3. Growth Factor: Every 3 hours, the count multiplies by 8.

💡 Strategic Tip: Growth factor = $\frac{\text{new value}}{\text{old value}}$ over one time period.

Choice B is incorrect because this would only give 100 → 200 → 400. Choice C is incorrect because this is $(\frac{6400}{100})$, not per period. Choice D is incorrect because this is the difference, not the ratio.