Picklists from a SFDC field will show up as values on a Marketo form if you change the field type of the field in the form to Select.
Only values common to both Lead & Contact/Account will appear in the Marketo picklist.
Changes to a picklist in SFDC will not change the picklist of an existing form in Marketo. If a picklist in SFDC changes a notification will appear in Marketo and the Marketo landing page will automatically move to draft. You will need to update each Marketo form and re-approve all landing pages using the form. Any new forms you create should show the updated picklist for your SFDC field.
Here is more information about updating a picklist field in SFDC.
Additional resources for learning about the Marketo integration with SFDC.
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