**Core Concept:** The larynx, also known as the voice box, is a cartilaginous and muscular structure that plays a crucial role in breathing, swallowing, and producing sound. It is composed of three primary parts: the epiglottis, the arytenoids, and the cricoid cartilage. Muscles of the larynx are responsible for maintaining its structure, protecting the airway, and facilitating vocalization.

**Why the Correct Answer is Right:** The correct answer, **D. Vocal Cord (Cordus Vocalis)**, is the only muscle located on the external aspect of the larynx. The vocal cords are responsible for producing sound during speech and singing. They are composed of two parallel folds of mucous membrane, covered with a smooth epithelium and separated by a narrow space called the vocal cleft.

**Why Each Wrong Option is Incorrect:**

A. **Arytenoid (Arytenoideus):** These are small, paired muscles that are attached to the arytenoid cartilages. They are located within the larynx, not on the external aspect.

B. **Cricothyroid (Cricothyroidus):** This muscle is a paired, slender muscle that connects the cricoid cartilage to the thyroid cartilage, but it is also located within the larynx, not on the external aspect.

C. **Apostrophus:** Also known as the thyroarytenoid muscle, it is a paired, thin muscle that is attached to the arytenoid cartilages and the thyroid cartilage, and is also internal to the larynx.

**Clinical Pearl:** The external laryngeal muscles include the cricothyroid and sternothyroid muscles, both of which aid in adjusting the tension of the vocal cords and controlling voice pitch. However, they are not associated with the vocal cords themselves.

**Correct Answer Explanation:** The correct answer, **D. Vocal Cord (Cordus Vocalis)**, is the only muscle that is specifically involved in the functioning of the vocal cords, which are crucial for producing sound during speech and singing. The vocal cords are located on the external aspect of the larynx, making option D the correct choice.