Explanation:

- The boat travels 56 km downstream in 3.5 hours, meaning its downstream speed is 56/3.5 = 16 km/h.

- The ratio of the speed of the boat in still water to the speed of the stream is 3:1. Let the speed of the stream be x km/h, then the speed of the boat in still water is 3x km/h.

- Downstream speed is (3x + x) = 4x km/h. So, 4x = 16 km/h implies x = 4 km/h.

- The speed of the boat in still water = 3x = 12 km/h.

- To find the time to cover 41.6 km downstream: downstream speed = 16 km/h.

- Time = Distance / Speed = 41.6 / 16 = 2.6 hours.

- This matches option 3. 2.6