Correct option is A
Total work = LCM (12,18,36) = 36 units
Efficiency of A = 36/12=3 units/hour
Efficiency of B = 36/36=1 units/hour
Efficiency of C = 36/18=2 units/hour
Let all the pipe is opened for X hours
ATQ,
(3-2+1) ×X + 7 × 4 = 36
2X = 8
X = 4
At 3:00 PM pipe C is closed.