Which muscle extends across the back of the shoulder and covers the back of the neck?

The trapezius muscle is the correct answer because it is a large, triangular muscle that extends across the back of the shoulder and covers the back of the neck. It has several functions, including stabilizing and moving the shoulder blades and supporting the arms. The trapezius can be divided into three distinct parts: the upper fibers, which elevate the shoulder; the middle fibers, which retract the shoulder; and the lower fibers, which depress the shoulder. Because of its location and the extent of its coverage on the upper back and neck, it plays a crucial role in facilitating movements of the head, neck, and shoulders.

In contrast, while the deltoid is associated with shoulder movements, it primarily covers the outer shoulder rather than the back of the neck. The pectoralis major is located in the front of the chest and is involved in movements of the arm rather than the back and neck area. The latissimus dorsi, on the other hand, is a large muscle in the lower back that primarily contributes to the movement of the shoulder and is not positioned to cover the neck and upper back area like the trapezius does.