Mila has a bag filled with marbles in which situations would the probability of mila drawing a blue marbles from the bag be 1-3

Since there were no situation given, I can only provide you with some guidance so you can select the correct answer.

If the probability is 1/3, that means that one out of every three options will need to be blue. 

If it is a bag of marbles, you could have 1 blue marble out of a total of 3 marbles. Or any other fraction that reduces to one third.

Here are some examples:

2/6   3/9   4/12   5/15   6/18   7/21   8/24