**Core Concept**
The pelvic diaphragm is a muscular and fibrous structure that separates the pelvic cavity from the perineal region. It is composed of two main muscle groups: the levator ani and the coccygeus muscles.

**Why the Correct Answer is Right**
The levator ani muscle is the primary muscle forming the pelvic diaphragm. It is a broad, flat muscle that arises from the inner surface of the ischial spine and sacrum, and inserts into the pubic bone and the perineal body. The levator ani muscle plays a crucial role in supporting the pelvic organs, including the bladder, uterus, and rectum, and helps to maintain continence. It is also involved in the regulation of pelvic floor movements, such as defecation and urination.

**Why Each Wrong Option is Incorrect**
**Option A:** **Incorrect**, as the obturator internus muscle is a part of the lateral wall of the pelvis, but not a primary component of the pelvic diaphragm.
**Option B:** **Incorrect**, as the iliacus muscle is a part of the iliopsoas muscle group, which is involved in hip flexion, but not a component of the pelvic diaphragm.
**Option C:** **Incorrect**, as the coccygeus muscle is a part of the pelvic diaphragm, but it is not the primary muscle forming it.

**Clinical Pearl / High-Yield Fact**
The pelvic diaphragm is a critical structure that supports the pelvic organs and maintains continence. Damage to this structure can lead to pelvic organ prolapse and urinary incontinence.

**Correct Answer:** C. Coccygeus muscle.