Choice A is the correct answer. Find the annual growth factor.

1. Formula: $S(t) = S_0(b)^t$ where $S_0 = 1$, $S(4) = 16$.
2. Solve: $16 = 1(b)^4$, so $b^4 = 16$.
3. Calculate: $b = \sqrt[4]{16} = 2$.
4. Verify: $1(2)^4 = 16$ ✓

💡 Strategic Tip: To find $b$ from $b^4 = 16$, take the 4th root.

Choice B is incorrect because this would give $(4)^4 = 256$. Choice C is incorrect because this is the total growth, not annual factor. Choice D is incorrect because this doesn't match $b^4 = 16$.