Choice A is the correct answer. Work backwards from the remaining percentage.

1. Remaining: 12.5% = $\frac{1}{8} = (\frac{1}{2})^3$.
2. Half-lives: 3 half-lives have occurred.
3. Half-life period: $\frac{15}{3} = 5$ days.
4. Verification: After 15 days = 3 periods: $(0.5)^3 = 0.125 = 12.5\%$ ✓

💡 Strategic Tip: 12.5% = $\frac{1}{8}$ means 3 halvings have occurred.

Choice B is incorrect because this would give 2 half-lives in 15 days. Choice C is incorrect because this would mean only 1.5 half-lives. Choice D is incorrect because this would mean 4 half-lives.