What type of plants flower only if the length of darkness is shorter than a certain period?

The correct choice is that long day plants flower only if the length of darkness is shorter than a critical period. These plants typically require longer daylight hours to initiate flowering. They are sensitive to varying lengths of darkness, and when the night is shorter than a specific duration, it signals to the plant that conditions are favorable for flowering. This response is part of a plant's photoperiodism – a physiological reaction to the length of day and night.

In contrast, short day plants require longer periods of darkness to flower, as they bloom when the nights are longer than a certain length. Diurnal and nocturnal plants describe the activity patterns of plants relative to day and night, but they are not defined by their flowering responses to photoperiod. Diurnal plants are active during the day, while nocturnal plants are active at night, but their flowering mechanisms are centered around the length of light exposure rather than just the activity cycle.