**Correct option: a) 1 km/hr**

**Explanation:**

Suppose he moves 4 km downstream in x hrs.

Then speed downstream = (4/x) km ph; and speed upstream =(3/x) km ph (48/(4/x))+(48/(3/x)) =14 therefore x= ½

Speed upstream= 6 km ph, speed downstream =8 km ph Rate of the stream=1/2*(8-6) = 1 km ph