Step 1: Setup:
Volume \(V = \frac43\pi r^3\) and surface area \(S = 4\pi r^2\). The condition is \(\frac{dV}{dt} = -kS\).
Step 2: Simplify:
\(\frac{dV}{dt} = 4\pi r^2\frac{dr}{dt}\). So \(4\pi r^2\frac{dr}{dt} = -k\cdot4\pi r^2\), giving \(\frac{dr}{dt} = -k\). The radius shrinks at a constant rate.
Step 3: Use the data:
The radius falls from 3 cm to 1 cm in 4 months, a drop of 2 cm, so the rate is 0.5 cm per month. To lose the full 3 cm at this rate takes \(\frac{3}{0.5} = 6\) months.
Final Answer:
The mothball evaporates completely in 6 months, option (A).
\[ \boxed{6\text{ months}} \]