Which of the following types of plants typically require long days to flower?

Long day plants are those that require extended periods of daylight to trigger flowering. These plants normally bloom when the day length exceeds a certain number of hours, typically around 12 to 16 hours of light. This adaptation allows them to take advantage of the longer days of spring and early summer, when environmental conditions are favorable for growth and reproduction.

In contrast, short day plants require longer nights to flower, as they typically bloom when the daylight hours decrease. Day neutral plants do not have strict day length requirements and can flower regardless of the amount of light they receive. Perennial plants can encompass a wide variety of flowering types, including long day, short day, and day neutral plants, but they are not defined by their photoperiod requirements alone.

Thus, long day plants are specifically characterized by their need for prolonged exposure to light, making them the correct choice in this context.