Best way to deal with multiple choice checkbox UX?

I know this is somewhat subjective, but I can't really make up my mind on the following UX issue (please ignore the design, so far the site is purely for functional testing)

I've got a site that acts like an a) b) c) quiz editor. One of the options is a toggle, whether multiple choices can be valid for an answer. By default this choice is disabled.

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Now, if the Multiple valid answersis disabled (by default), if you 'check' an answer in a question, the one that was previously marked as true will be replaced by the new one.

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Now, with Multiple valid answers enabled:

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I feel like it might be more intuitive to have it on by default, but then it feels super weird having one option on by default and rest off.

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So now I'm basically stuck in the middle, where I like features from either side, but both have their downsides as well which turn me off.

I guess having all the toggles off seems like the lesser of two evils.

I also thought about potentially letting users select multiple choices and only displaying an error message on submit, in case the multiple valid answers field is disabled, but that just feels like an UX oversight instead.

What's your opinion? :-)
PS: Happy holidays!

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