How do I create a conditional form field?
You can do this using taxonomy terms.  A warning though ... taxonomy worked terribly due to the amount of conditional items.  For my situation, user selected a state, then the town it took about a minute for each selection.    

Does this help?  http://www.webbykat.com/2013/04/conditional-fields-and-term-reference-tree-two-small-bugs-their-resolution

I'm assuming you have this Drupal 7 module?  https://www.drupal.org/project/conditional_fields