Custom Field Multiple Choice Through the API?
Yes, if you visit your api explorer you'll be able to play with this in detail, but the api call you probably want is:

civicrm_api3('contact', 'getoptions', array('field' =&gt; 'custom_123'));


where 123 is the id of your custom field.

By the way, you haven't said how/where you are developing this form, but if it's on your own site with access to the ajax api, then a big shortcut for you would be to simply drop in an entityRef field (entity type = "option_value") and given the correct option group it will do everything else for you.