This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Anatomy
Sternocleidomastoid muscle is supplied by: 
Select and explain. 
A. Vagus nerve, B. Spinal accessory nerve, C. Marginal mandibular branch of facial nerve, D. None of the above
B: Spinal accessory nerve
Answer: B: Spinal accessory nerve 
Explanation: The spinal accessory nerve provides the motor supply. It passes through the muscle.
The spinal accessory nerve supplies:

The sternocleidomastoid, the chin turning
The trapezius, the shrugging muscle.

Cervical nerves provide a proprioceptive sensations to these muscles.
Reference: B D Chaurasia’s Head and Neck anatomy, 6th ed page no 384