Answer:
a. VIP (vasoactive intestinal peptide)
Step-by-step explanation:
The enteric nervous system (ENS) is one of the main divisions of the autonomic nervous system (ANS) together with sympathetic and parasympathetic nervous system. This system consists of neurons that regulate the function of the gastrointestinal tract (GI).
Inhibitory interneurons that are part of the enteric nervous system also known as Dogiel type I neurones achieve their inhibitory transmission via nitric oxide, adenosine triphosphate, vasoactive intestinal peptide (VIP) and pituitary activating cyclic AMP. They usually act directly on smooth muscles of GI.