Hey! Without being able to see your designs, I think the easiest option would be to use the question copy. You can do this by a) writing the question so that it implies that you have to select more than one, b) literally having the words ‘select multiple’ at the end of the question — in different styling and colour.

Which puppy is the cutest?
vs
Select which puppies are the cutest. (Select multiple)

Alternatively, you could have a checkbox square next to the label of each item to indicate that it is a multi-select. But I suspect that this, won’t be clear enough.

If you and your team have a better idea, please let me know!