Explanation:

The problem can be solved by traditional method but it is a cumbersome process to do that

The problem is simple if its solution is simpler. Hence, we will go for a simpler solution for this kind of problem.
There is a shortcut method to find the Quantity of milk left after nth operation
It is given by (x−y/x)ⁿ of the whole quantity, where x is the initial quantity of milk in the cask y is the quantity of milk withdrawn in each process and n is the number of processes.
Hence from the above rule it can be say that, 

Quantity of milk left after the 3rd operation =30×(30−6/30)ⁿ =15.36 litres.

Hence, option 4 is the correct answer.