Final answer:
The subclavius muscle is innervated by a nerve branch that comes from the brachial plexus, specifically from the C5 and C6 spinal nerves.
Step-by-step explanation:
The nerve that innervates the subclavius muscle is derived from the brachial plexus, which is associated with the lower cervical spinal nerves and the first thoracic spinal nerve. Specifically, the subclavius is innervated by a nerve branch that originates from the union of the C5 and C6 spinal nerves. Therefore, the correct answer to the question 'What is the nerve to subclavius innervated by?' is 1) C5 and 2) C6.