In this question, there is nothing complicated. only thing that needs to be done is to convert 30 liters into milliliters. For this purpose it is important to know the number of milliliters in 1 liter. then the problem becomes very easy to solve.
1 liter = 1000 milliliters
then
30 liters = (1000/1) * 30 milliliters
= 1000 * 30 milliliters
= 30000 milliliters
So from the above deduction we can conclude that the third option of 30000 is the correct option,. i hope there is nothing uncomplicated in the procedure for you to understand.