**Core Concept**
The question is testing the student's knowledge of the anatomy and function of the muscles of the shoulder joint. The movements of abduction and internal rotation of the arm are complex and involve the coordinated action of multiple muscles.

**Why the Correct Answer is Right**
The correct answer is the subscapularis muscle, which is a part of the rotator cuff. The subscapularis muscle is responsible for internal rotation of the arm, and it also assists in abduction of the arm. The subscapularis muscle originates from the subscapular fossa of the scapula and inserts into the lesser tubercle of the humerus. During abduction, the subscapularis muscle helps to rotate the humerus externally, which is essential for full range of motion. In internal rotation, the subscapularis muscle contracts to rotate the humerus medially.

**Why Each Wrong Option is Incorrect**
**Option A:** The supraspinatus muscle is primarily responsible for abduction of the arm, but it is not primarily responsible for internal rotation.
**Option B:** The infraspinatus muscle is also a part of the rotator cuff and assists in external rotation of the arm, but it is not primarily responsible for abduction or internal rotation.
**Option C:** The teres minor muscle assists in adduction and external rotation of the arm, but it is not primarily responsible for abduction or internal rotation.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) work together to provide stability and mobility to the shoulder joint. Dysfunction of any one of these muscles can lead to shoulder pathology.

**Correct Answer:** C. Subscapularis muscle.