In still water, a boat can cover a distance of 10 km at the speed of 5 km/h in certain time interval and during the same time interval it can cover a distance of 8 km while moving upstream in a river. What is the speed of the river (in km/h)?

In still water, the boat can cover a distance of 10 km with a speed of 5 km/hr in $$\frac{10}{5}=2$$ hours.

In two hours, the boat travels 8 km upstream. 
Therefore, the speed of boat upstream is $$\frac{8}{2}=4$$km/hr.

We know the speed of the boat is 5 km/hr and thus we can find the speed of the stream.

In upstream, effective speed = boat's speed - stream's speed
4 = 5 - stream's speed
Stream's speed = 1 km/hr

Hence, Option C is the correct answer.