The correct answer is a) trapezius.
The trapezius muscle plays a crucial role in stabilizing and anchoring the scapula, which is essential for various shoulder movements. It is a large muscle that extends from the back of the head and down the spine to the middle of the back, attaching to the scapula and clavicle. Its various segments are responsible for lifting, rotating, and stabilizing the scapula during arm movements.
While the other muscles mentioned, such as the levator scapula, serratus anterior, and latissimus dorsi, contribute to the movement and stability of the shoulder girdle, the trapezius is specifically key to anchoring the scapula to the thoracic spine, allowing for a strong foundation for the upper limb movements.