O C. A sweater sale on the hottest day of the year.
Explanation:
Irony is a figurative language used to express one thing but the opposite happens. It is used mostly in humor and satirical expressions when one wants to criticize.
Among the given sentences, the most clearly ironic sentence is option C. Selling of sweaters is supposed to be an event seen during cold climates. So, selling warm clothes on a hot sunny day is ironic. It's the total opposite of what is the most probable scenario.
