Explanation:

- The time taken by a boat to travel a certain distance is inversely proportional to its speed.

- Given that the time ratio of Boat A to Boat B:

- Upstream: 2:1, indicates Boat B is twice as fast as Boat A upstream.

- Downstream: 3:2, implies Boat B is 1.5 times as fast as Boat A downstream.

- Let the speed of Boat B in still water be $V$, and the speed of the stream be $S$.

- The speed downstream for Boat B is $V+S$ and upstream is $V-S$.

- Solving these relations, it turns out that $S/V=1/5$, or 20%.

Correct Answer: Option 3 – 20%.