Maël mixes 15 milliliter (ml) of bleach with 3.75 liters (l) of water to make a sanitizing solution for a daycare. the amounts of bleach and water always have to be proportional when he makes sanitizing solution. which of the following could be combinations of volumes of bleach and water for maël's sanitizing solution?

You have not provided the options, therefore, I cannot give you an exact answer.
However, I will try to help you with the concept.

First, we need to make all units the same in the problem. I will convert all units to ml.
We have:
amount of bleach = 15 ml
amount of water = 3.75 liters = 3.75 * 1000 = 3750 ml

Now, we will get the ratio between bleach and water:


This ratio is fixed, therefore, to get the right choice:
1- make all units the same
2- for each option, divide the amount of bleach by the amount of water.
The correct choice would be the one giving you 

Hope this helps :)