Explanation:

- Downstream speed is the speed of the boat with the current.

- Upstream speed is the speed of the boat against the current.

- The difference between them is 4 kmph.

- Let the speed of the current be \( x \) kmph.

- Speed of the boat in still water is \( 7x \) kmph.

- Downstream speed = \( 7x + x = 8x \) kmph.

- Upstream speed = \( 7x - x = 6x \) kmph.

- Given the difference: \( 8x - 6x = 2x = 4 \) kmph. Therefore, \( x = 2 \) kmph.

- Thus, downstream speed = \( 8 \times 2 = 16 \) kmph.

- Time to cover 48 km downstream = \( \frac{48}{16} = 3 \) hours.

- So, the correct option is Option 5: 3 hours.