This means that 'optional' needs to somehow be integral to the plot of your story.  We would recommend looking up the definition(s) of the word in a reputable dictionary first.  You then need to make sure that one (or more) of these definitions is evident and important in your story.  For example, a character could be presented with something that is optional and their choice could play an important role in the story.  There are many other ways 'optional' could be integrated into a story, but we hope this helps.  Also, just to be clear, it does not mean the subject is open.  Please let us know if you have any other questions or need anything else.  Good luck!