Explanation:

- Boat's speed in still water: 48 km / 3 hours = 16 km/hr.

- Speed of current: 4 km/hr.

- Upstream speed: 16 km/hr (boat speed) - 4 km/hr (current speed) = 12 km/hr.

- Time taken upstream: 48 km / 12 km/hr = 4 hours.

- Time taken in still water: 3 hours.

- Difference in time: 4 hours - 3 hours = 1 hour.

- Correct Answer: Option 4 - 1 hour.

The calculations show the difference in time is 1 hour, matching your answer.

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