## Core Concept
The movement of abduction at the shoulder joint involves the coordinated action of multiple muscles. The primary muscles responsible for shoulder abduction are the **supraspinatus** and **deltoid**. The supraspinatus initiates the abduction from 0 to 15 degrees, while the deltoid takes over to continue the movement from 15 to 90 degrees.

## Why the Correct Answer is Right
The muscles primarily responsible for the abduction of the shoulder are the **supraspinatus** and **deltoid**. The **supraspinatus** muscle is responsible for the initial 15 degrees of abduction due to its anatomical position and line of pull. The **deltoid** muscle then continues the abduction up to 90 degrees. The **trapezius** and **serratus anterior** also play supportive roles in upward rotation and stabilization of the scapula during the abduction movement, especially beyond 90 degrees. However, the **pectoralis major** primarily functions in adduction, medial rotation, and flexion of the shoulder joint, making it not a primary abductor.

## Why Each Wrong Option is Incorrect
- **Option A:** The **supraspinatus** is a correct muscle involved in shoulder abduction, specifically initiating the movement.
- **Option B:** The **deltoid** is also correct as it continues the abduction of the shoulder.
- **Option D:** The **trapezius** assists in upward rotation of the scapula, which is crucial for the abduction movement beyond 90 degrees.

## Why Option C is Incorrect
- **Option C:** The **pectoralis major** primarily functions in movements that are opposite to abduction, such as adduction, and is not considered a primary muscle for shoulder abduction.

## Clinical Pearl / High-Yield Fact
A key clinical point to remember is the initiation of shoulder abduction. The **supraspinatus** must function properly to initiate abduction. Weakness in this muscle, as seen in rotator cuff injuries, can significantly impair the ability to abduct the shoulder.

## Correct Answer: C. pectoralis major.