Pipe A alone can fill a tank in 8 hours. Pipe B alone can fill it in 6 hours. If both the pipes are opened and after 2 hours pipe A is closed, then the other pipe will fill the tank in

Let us consider the capacity of the tank to be 48 litres.
Pipe A fills it in 8 hours at 6 litres per hour.
Pipe B fills it in 6 hours at 8 litres per hour.
Total of 28 litres would be filled in 2 hours with both pipes open.
Afetr 2 hours Pipe A is closed.Hence Pipe B has to fill the remaining tank.
Quantity of water needed to fill the tank is =48-28 = 20 litres.
Pipe B fills at the rate of 8 litres per hour.
Hence it takes 2.5 hours for Pipe B to fill remaining 20 litres of the tank.
Hence Option D is the correct answer.