Explanation:

- The speed of the boat in still water is 6 km/h.

- If going upstream takes four times the downstream time, there's a speed differential due to the stream.

- Let \( x \) be the speed of the stream.

- Downstream speed: \( 6 + x \) km/h

- Upstream speed: \( 6 - x \) km/h

- The time ratio implies that \( 6 - x = \frac{1}{4} \times (6 + x) \).

- Solving \( (6 - x) = \frac{1}{4} \times (6 + x) \) leads to \( x = 3.6 \) km/h.

- Therefore, the correct speed of the stream is 3.6 km/h.

- Option 2: 3.6 km/h is the right answer.