Correct Answer - Option 4 : 6 km

**Given **

The boat covers a distance from A to B downstream and from B to A upstream in 4 hours

The speed of the stream = 2 km/hr

The speed of the boat in still water = 4 km/hr,

**Concept Used:**

Upstream speed = Speed of boat - Speed of the stream

Downstream speed = Speed of boat + Speed of the stream

**Calculation :**

Speed of boat downstream = (4 + 2) km/hr = 6 km/hr

speed of boat upstream = (4 - 2) km/hr = 2 km/hr

Let, the distance between point $A$ and point $B$ be $x$ km

Accordingly,

x/6 + x/2 = 4

⇒ (x + 3x)/6 = 4 (cross multiple)

⇒ 4x = 24

⇒ x = 24/ 4

⇒ x = 6

**∴ The distance between A and B is 6 km**