Concept: The question tests on the area of Rates(Time and Work)
Solution:
Pipe A Takes 2 hours to fill the tank alone.
Work done in 1 hour = 1/2
Let the Leak take x hours to empty the tank.
In 1 hour, work done = 1/x
Pipe A filling and the leak emptying take 2.5 hours to fill the tank.
Work done in 1 hour = 1/2.5 = 2/5
Therefore 1/2−1/x=2/5
1/2−2/5=1/x
5−4/10=1/10=1/x
x = 10 hours
Option D