Step 1: Photoperiodism is the physiological response of plants to the relative lengths of day and night, mainly affecting flowering time.
Step 2: Textbooks recognize exactly three response groups: short-day plants like chrysanthemum and rice, long-day plants like wheat and spinach, and day-neutral plants like tomato and cucumber whose flowering does not depend on photoperiod at all.
Step 3: Checking each option against this list, short-day, long-day and day-neutral are all genuine categories, leaving mid-day plants as the term that does not exist in botany.
Final answer: Mid-day plants.