Concept: The question tests on basics of Rates(Time and Work)
Solution:
First pipe - Fills 1/2 tank in 3 hours =>1 full tank in 6 hours
Second pipe- Fills 2/3 rd tank in 6 hours = > 1 full tank in 6 * (3/2) = 9 hours
LCM(6,9) = 18 => Let the capacity of the tank be 18 units
=> Efficiency of first pipe = 18/6 = 3 units/hour
=>Efficiency of second pipe = 18/9 = 2 units/hour
Working together, they fill 3+2 =5 units/hour
=> They would take 18/5 = 3.6 hours (option b)