Choice A is the correct answer. Perform the exponentiation first.

1. Exponent: $(0.5)^3 = 0.5 \times 0.5 \times 0.5 = 0.125$.
2. Multiply: $100 \times 0.125 = 12.5$.

💡 Strategic Tip:$0.5$ is $1/2$. $(1/2)^3 = 1/8$. $100/8 = 12.5$.

Choice B is incorrect because it represents 2 half-lives ($100 \to 50 \to 25$). Choice C is incorrect because it represents 1 half-life.