Final answer:
The most important nerve of the cervical plexus that innervates the diaphragm is the Phrenic nerve.
Step-by-step explanation:
The most important nerve of the cervical plexus, which innervates the diaphragm, is the Phrenic nerve. The cervical plexus is composed of axons from spinal nerves C1 through C5 and branches into nerves in the posterior neck and head, as well as the phrenic nerve, which connects to the diaphragm at the base of the thoracic cavity.