Correct option is C
Let total capacity of the tank = 60 unit (LCM of 15, 60, 10)
Now, efficiency of the first, second and third pipe be 4 unit/minute, 1 unit/minute and 6 units/minute respectively.
Tank filled in first 10 minutes = (4+1)×10=50 unit
Now, when all the pipe work together, 1 unit of water will out in every minute from tank.
So, 50 units of water will be emptied in 50 minute