Each day, a bean plant holds its leaves horizontally during the day but folds them close to its stem at night. This is an example of ____.

A) a photoperiodic response
B) vernalization
C) a circadian rhythm
D) abscission
E) systemic acquired resistance

The daily movement of the plant's leaves is an example of a circadian rhythm, which is a biological process that repeats in a 24-hour cycle. Photoperiodic responses relate to changes in daylight hours, vernalization is a response to cold temperatures, abscission is the shedding of leaves, and systemic acquired resistance is a plant's ability to defend itself against pathogens. None of these options are applicable to the daily movement of a bean plant's leaves.