Pipe A and Pipe B fill water into a tank of capacity 1000 litres at

At time 0 minutes, water in the tank = 0 liters

At time 1 minute, water in the tank = 100 liters (0 from before + 100 from pipe A)

At time 2 minutes, water in the tank = 125 liters (100 from before + 25 from pipe B)

At time 3 minutes, water in the tank = 75 liters (125 from before - 50 from pipe C)

If we take these steps as one single step.

At time 6 minutes, water in the tank = 150 liters (75 from before + 75 from the new round)

Now, we can see that after 6*6 = 36 minutes, water in the tank = 150 * 6= 900 liters

Now, since we start the process over again, At time 37 minutes, water in the tank = 1000 liters (900 from before + 100 from pipe A)

Hence D

Hope that makes it clear