Explanation:

- The man's rowing speed in still water is 5 km/h.

- The current's speed is 2.5 km/h.

- In downstream, the effective speed is 5 km/h + 2.5 km/h = 7.5 km/h.

- In upstream, the effective speed is 5 km/h - 2.5 km/h = 2.5 km/h.

- Let the distance be "d" km.

- Time taken downstream = d / 7.5 hours

- Time taken upstream = d / 2.5 hours

- It takes 4 hours more to travel upstream, so:

$$ \frac{d}{2.5} = \frac{d}{7.5} + 4 $$

- Solving, \(3d = d + 30\), thus, \(2d = 30\), so \(d = 15 \text{ km}\).

15 km is the correct answer.