Explanation:

Speed of the boat upstream =9–3 =6 km/hr

Speed of the boat downstream=9+3=12 km/hr

Let's distance between A and B be x kms . 

So,time taken to travel x km upstreamby boat= x/6 hrs.

And time taken to travel x kms downstream= x/12 hrs.

According to the question ,

x/6 + x/12 =3

x[1/6 + 1/12]=3

x = 4×3 =12

So, distance between A and B is 12 kms.

Hence, option 4 is the correct answer.