Allow Author to Build Multiple Choice Question?
There isn't a field type or plugin I know of that will let an admin choose only 1 correct answer.

One possibility would be to number the correct answer in another field like piotrpog suggests. I would argue this is possibly more confusing and brittle since an admin could accidentally update the answer choices without updating the correct answer field. The two fields aren't linked automatically.

I think it's pretty reasonable to ask admin's to only mark one choice as correct. You could also reinforce this in the field instructions. The lightswitch field makes it pretty easy to scan the column to make sure.