**Core Concept**: Denervation refers to the loss of motor neuron connections to a muscle, leading to muscle weakness or paralysis. In this case, we are discussing a deficit in muscle function of the right upper limb, specifically the inability to abduct the arm. This indicates that the muscle concerned is part of the upper limb abduction movement.

**Why the Correct Answer is Right**: The correct answer, **D**, is the deltoid muscle, which is a key muscle involved in shoulder abduction and elevation, essential for arm abduction. It is a large, triangular muscle situated in the anterior aspect of the shoulder joint, originating from the acromion process, coracoid process, and the lateral aspect of the scapula, and inserting onto the humerus. When the deltoid muscle is denervated, it leads to the inability to abduct the arm, as observed in the question.

**Why Each Wrong Option is Incorrect**:

A. The supraspinatus muscle is involved in rotator cuff movements, not abduction. So, it is incorrect in this context.
B. The infraspinatus muscle is part of the rotator cuff and does not contribute to arm abduction, making it an incorrect choice.
C. The subscapularis muscle is involved in internal rotation and flexion of the shoulder joint, not abduction, so it is not the correct answer.

**Why the Correct Answer is Right**: The correct answer, **D**, is the deltoid muscle, which plays a crucial role in shoulder abduction and elevation. Its denervation results in the observed deficit in muscle function, specifically arm abduction.

**Clinical Pearl**: Understanding denervation and its impact on muscle function is essential for diagnosing and treating neuromuscular disorders. Deltoid muscle weakness can be caused by various factors, including trauma, tumors, infections, or autoimmune diseases. In clinical practice, evaluating shoulder abduction and considering muscle denervation can aid in formulating a proper diagnosis and treatment plan for patients with similar symptoms.