Final answer:
The muscle affected in congenital torticollis is the sternocleidomastoid, which is responsible for the lateral flexion and rotation of the head. The correct answer is option (B).
Step-by-step explanation:
Babies born with congenital torticollis are unable to rotate their heads properly and their necks are twisted to one side because the muscle that is affected in congenital torticollis is the sternocleidomastoid muscle. The sternocleidomastoid is the major muscle that laterally flexes and rotates the head. It divides the neck into anterior and posterior triangles when viewed from the side and is controlled by the accessory nerve, which also innervates the trapezius muscle.