**Core Concept**
The pelvic diaphragm is a muscular layer that separates the pelvic cavity from the perineum. It is composed of two main muscles: the levator ani and the coccygeus. These muscles work together to provide support to the pelvic organs and maintain continence.

**Why the Correct Answer is Right**
The coccygeus muscle, also known as the ischiococcygeus muscle, plays a crucial role in the formation of the pelvic diaphragm. It is a broad, flat muscle that arises from the ischial spine and inserts into the lateral border of the coccyx and the gluteal fascia. The coccygeus muscle, along with the levator ani, helps to elevate the pelvic floor and maintain the integrity of the pelvic diaphragm. This muscle is innervated by the pudendal nerve and is essential for maintaining continence and supporting the pelvic organs.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the obturator internus muscle is not a primary component of the pelvic diaphragm. It is a small muscle that arises from the inner surface of the obturator membrane and inserts into the greater trochanter of the femur.
**Option B:** Incorrect because the piriformis muscle is not a part of the pelvic diaphragm. It is a broad, flat muscle that arises from the anterior surface of the sacrum and inserts into the greater trochanter of the femur.
**Option C:** Incorrect because the iliococcygeus muscle is a part of the levator ani muscle group, but it is not a separate muscle that forms the pelvic diaphragm along with the levator ani.

**Clinical Pearl / High-Yield Fact**
The pelvic diaphragm is a critical structure that plays a vital role in maintaining continence and supporting the pelvic organs. Damage to this muscle can lead to pelvic organ prolapse and urinary incontinence.

**Correct Answer: C. Coccygeus**