The correct answer is (b).
If 1/4 of a water tank is filled in 15 minutes, then the rest of the tank, which is 3/4 of the tank, will be filled in 65 minutes. This is because 1/4 of the tank is filled in 15 minutes, and 3/4 of the tank is 3 times as much as 1/4 of the tank. Therefore, 3/4 of the tank will be filled in 3 times as much as 15 minutes, which is 65 minutes.
Option (a) is incorrect because 61 minutes is less than 15 minutes, which is the time it takes to fill 1/4 of the tank. Option (c) is incorrect because 79 minutes is more than 65 minutes, which is the time it takes to fill 3/4 of the tank. Option (d) is incorrect because 81 minutes is more than 65 minutes, which is the time it takes to fill 3/4 of the tank.