Photoperiods are believed to be perceived by

The correct option is B Pineal body

• Photoperiodism is the physiological reaction to the length of day or night by an organism.
  
• The pineal gland or pineal body (epiphysis or third eye) is a small endocrine gland.
• It secretes melatonin (hormone) which regulates the wake/sleep patterns and photoperiodic functions.
  
• So, the correct answer is 'Pineal body'.