Explanation:

We're asked to find the time t it takes a boat to travel 68 km.

speed of boat with respect to earth: 13 km/h

speed of stream with respect to earth: 4 km/h

We're given that the boat travels downstream, i.e. with the current, so the new speed of the boat is

13 km/h +4 km/h =17 km/h

The time to travel 68 km with this speed is thus

t=68km/17kmh=4 h

Hence, option 3 is the correct answer.