Thanks for reaching out. This is not currently a capability the platform has, but it is on our roadmap for a future update. Currently, you can collect short answer responses for your custom fields.
In this future update, we're planning to add a bunch more options for custom fields including form options like drop downs, checkboxes, multiple choice answers, and more.
I don't have a specific timeline yet for when this will be released, but keep an eye on updates.rallyup.com to be one of the first to know along with keeping up to date on all of our latest product updates.