Set up a related table of records. Set up a portal with buttons such that the portal lists all the response options, each in a different record and clicking a button in the portal row can create or delete a related record. With conditional formatting and/or FileMaker 13's new "hide when" feature, you can set up the button so that it looks and acts like a check box.
But now your responses are recorded as a series of related records and this allows you to count each response separately.
Thanks, PhilModJunk! That actually makes perfect sense.