The brachial plexus is a network of nerves formed by the anterior rami of the lower four cervical nerves and first thoracic nerve (C5, C6, C7, C8, and T1). This plexus extends from the spinal cord, through the cervicoaxillary canal in the neck, over the first rib, and into the armpit. It supplies afferent and efferent nerve fibers to the chest, shoulder, arm and hand.

Answer this question based on the passage: where do all nerves in the arm root from? OK. Let's think. My stream of consciousness:
This is the relevant information: It supplies afferent and efferent nerve fibers to the chest, shoulder, arm and hand. The answer is brachial plexus.