Our Consent to Processing field and Consent to Direct Marketing field are both string (picklist) with "Yes" and "No" as possible values—but we don't use Marketo forms so we don't have the same restrictions. You should probably do boolean (checkbox) to get it to show.
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The way you store the fields (field type) is distinct to the way the field is represented in the forms (can be a droplist or a checkbox).
It's strongly recommended that the field type in Marketo and in SFDC be the same, otherwise, you will end up with some serious sync issues.