What is photoperiod how does it affect plants

The correct option is A).
Photoperiod is the duration of light which is required by a plant for flowering. It can also be called as day length. It varies with different zones on earth and also according to seasons.

Photoperiodism is the biological response of an organism whether plants or animals towards the  change in intensity of light during day and night periodically. The response usually is observed by the changing behavior or growth of the organism.
Some plants require periodic exposure to light to induce flowering.

• Long Day Plants − Plants that require exposure to light for a period exceeding a well defined critical duration to induce flowering

• Short Day Plants − Plants that require exposure to light for a period less than this critical period to induce flowering

• Day Neutral Plants − Plants where there is no correlation between exposure to light duration and induction of flowering