Choice B is the correct answer. Check initial value and ratio.

1. Initial Value: At $x=0, y=10$. So $a=10$.
2. Ratio: $5/10 = 0.5$, $2.5/5 = 0.5$. So $b=0.5$.
3. Equation: $y = 10(0.5)^x$.

💡 Strategic Tip: If $y$ is getting smaller by division, it's decay ($b < 1$).

Choice A is incorrect because it represents doubling. Choice D is incorrect because it is linear subtraction.