Question 1005689
The answer is, a container of 8 lit.

The 24 lit container will be filled in 3 times, the 56 lit. container in 7 times, and 120 lit. in 15 times. 

For the largest container that can be used to fill them all, it has to be <24, and should provide an integer solution when 56 & 120 are divided as well, which 24 does not satisfy.
The top 2 largest containers that will fill 24 lit are 12 & 8 lit. Of these, 56/12 is not an integer but 56/8=3 is, and so is 120/8=15.