Explanation:

To determine the speed of the stream, we can analyze the problem as follows:

- Upstream Travel: The boat travels 21 km in 3 hours.

- Speed upstream = Distance / Time = 21 km / 3 h = 7 km/h.

- Downstream Travel: The boat travels 55 km in 5 hours.

- Speed downstream = Distance / Time = 55 km / 5 h = 11 km/h.

- Boat's Speed:

- Let \( b \) represent the speed of the boat in still water, and \( s \) represent the speed of the stream.

- Upstream: \( b - s = 7 \).

- Downstream: \( b + s = 11 \).

- Calculate Speed of Stream:

- Add both equations: \( (b - s) + (b + s) = 7 + 11 \).

- This simplifies to: \( 2b = 18 \).

- Therefore, \( b = 9 \).

- Substitute back: \( 9 - s = 7 \).

- Solve for \( s \): \( s = 2 \).

- Correct Option:

- 4. 2 km/h