The "undecided" option on a Yes/No field is set by default to indicate that the field has not been filled out yet. This is done to prevent users from accidentally submitting a review with an automatically selected value in error.

When a reviewer attempts to submit a form with "undecided" selected, the form will return the error "This Review may not be submitted until a response has been provided in all feedback prompts". This is  intentional, as Yes/No fields can only support "Yes" or "No" as their response.