Correct option is A
X, Y and Z can complete a piece of work in 20,12 and 28 days.
Work done = time × efficiency
Total work = LCM(20,12, 28) = 420
Efficiency of X = 420/20 = 21
Efficiency of Y = 420/12 = 35
Efficiency of Z = 420/28 = 15
Time taken by (X + Y + Z) together = 420/(21 + 35 + 15) = 5.91