Answer:
a. gastrointestinal system
Explanation:
Serotonin is a neurotransmitter that acts in the brain, establishing communication between nerve cells and can also be found in the gastrointestinal system and blood platelets. This hormone is produced through an amino acid called tryptophan, which is obtained through food.
Serotonin is a hormone that acts by regulating mood, sleep, appetite, heart rate, body temperature, sensitivity and intellectual functions and therefore, when this hormone is in a low concentration, can cause moodiness, difficulty sleeping, anxiety or even depression.