Emptying the tank is nothing but filling some number of buckets as determined the rate of filling of the pipes A,B and C..So this is a reverse question basically , reverse in the sense that time taken to empty the tank is given so we are given the time interval for which the pipes operate which is 2 hours = 120 minutes as mentioned in the question
Given filling rate of pipe A = 4 buckets in 24 minutes
Hence number of buckets A will fill in 120 minutes = 4 * 5 = 20 buckets = 100 litres [ As 1 bucket = 5 litres given ]
Filling rate of pipe B = 8 buckets in 60 minutes
Hence number of buckets B will fill in 120 minutes = 8 * 2 = 16 buckets = 80 litres
Filling rate of pipe C = 2 buckets in 20 minutes
Hence number of buckets C will fill in 120 minutes = 2 * 6 = 12 buckets = 60 litres
Thus total water being fed to buckets by pipe A , B and C = 100 + 80 + 60 = 240 litres
Hence this is nothing but the capacity of the tank..
Hence B) should be the correct option.