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 .....