Choice A is the correct answer. Use half-life calculation.

1. Half-lives: $\frac{12}{4} = 3$ half-lives.
2. Remaining: $600 \times (\frac{1}{2})^3 = 600 \times \frac{1}{8} = 75$ mg.
3. Formula: $A(t) = 600(0.5)^{t/4}$.

💡 Strategic Tip: 3 half-lives = $\frac{1}{8}$ of original amount.

Choice B is incorrect because this is after 2 half-lives (8 hours). Choice C is incorrect because this is after 1 half-life (4 hours). Choice D is incorrect because this would be after 4 half-lives (16 hours).