Final answer:
'Letters' is the category that does not belong among the others since it is not a form of fictional writing while 'Plays,' 'Novels,' 'Short Stories,' and 'Mysteries' are.
Step-by-step explanation:
Analyzing the headings: 'Plays,' 'Novels,' 'Short Stories,' 'Letters,' and 'Mysteries,' we can determine which category does not belong. Given that 'Plays,' 'Novels,' and 'Short Stories' are all forms of fictional writings and considering that Dorothy L. Sayers is known for her mysteries which are a genre of fiction, we can conclude that 'Letters' does not belong since it is not a fictional genre but a form of non-fictional correspondence.