**Core Concept**
The movement of the scapula, specifically its retraction at the sternoclavicular joint, involves the coordinated action of various muscles that attach to the scapula. This movement is essential for the scapula to move posteriorly and inferiorly, which in turn facilitates movements such as adduction and internal rotation of the arm.

**Why the Correct Answer is Right**
The trapezius muscle plays a crucial role in the retraction of the scapula at the sternoclavicular joint. This muscle originates from the external occipital protuberance and the superior nuchal line of the occipital bone, and inserts into the lateral third of the clavicle, the acromion, and the spine of the scapula. When the trapezius contracts, it pulls the scapula posteriorly and inferiorly, allowing for the retraction of the scapula at the sternoclavicular joint. This movement is essential for activities such as reaching behind the back or pulling objects towards the body.

**Why Each Wrong Option is Incorrect**
**Option A:** Serratus anterior is primarily responsible for the forward movement of the scapula (protraction) and upward rotation, rather than retraction. It originates from the surface of the upper border of the 1st to 8th ribs and inserts into the costal surface of the scapula.
**Option C:** Subscapularis is a rotator cuff muscle that helps in medial rotation and adduction of the arm, but it does not contribute to the retraction of the scapula at the sternoclavicular joint. It originates from the subscapular fossa and inserts into the lesser tubercle of the humerus.
**Option D:** Supraspinatus is another rotator cuff muscle that helps in abduction of the arm, but it does not contribute to the retraction of the scapula at the sternoclavicular joint. It originates from the supraspinous fossa and inserts into the superior facet of the greater tubercle of the humerus.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the trapezius muscle also plays a role in upward rotation and depression of the scapula, in addition to retraction. This can be remembered using the mnemonic "TRAP": Trapezius (R)otates (A)nd (P)ulls the scapula (downward and posteriorly).

**✓ Correct Answer: B. Trapezius**