Hello @LeticiaF76 ,
It looks like you may have already spoken with a phone agent regarding this. For the sake of visibility however, at the moment there isn't really a way to affect the thank you page showing up / make the poll block be multiple choice. We do have a feature request our engineers are looking into, so I've gone ahead and tracked your feedback for them. If and when they have an update to this request, they'll make sure to notify you.
In the meantime, you're free to use either our internal Survey tool to allow for multiple choice selections, or you can try linking to a poll created in a third party program that allows for multiple selection.