Photoperiodism is a biological response exhibited by organisms in response to the duration of light and darkness in a day. It is not limited to plants, as option 1 suggests. Instead, photoperiodism is observed in a wide range of organisms, including animals and even bacteria. Option 2 correctly states that photoperiodism is related to flowering in plants. This means that the initiation of the flowering process in plants is influenced by the length of the day and night. Option 3, which suggests that photoperiodism is related to feathers of animals, is incorrect. Feathers are not directly involved in the photoperiodic response. Therefore, option 4 is the correct answer as it accurately states that photoperiodism is the reaction of organisms to the length of day or night.