Choice A is the correct answer. Calculate using half-life periods.

1. Half-lives: $\frac{18}{6} = 3$ half-lives.
2. After each half-life: Multiply by 0.5.
3. After 3 half-lives: $400 \times (0.5)^3 = 400 \times 0.125 = 50$ mg.
4. Formula: $A(t) = 400(0.5)^{t/6}$.

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

Choice B is incorrect because this is after 2 half-lives (12 hours). Choice C is incorrect because this would be after 4 half-lives (24 hours). Choice D is incorrect because this is after only 1 half-life (6 hours).