**Core Concept**
The levator ani muscle is a crucial component of the pelvic diaphragm, playing a vital role in maintaining pelvic organ support and continence. It is a broad, flat muscle that originates from the arcus tendineus levator ani and inserts into the pubic bone, coccyx, and the anococcygeal ligament.

**Why the Correct Answer is Right**
The correct answer is related to the anatomical level of the levator ani muscle, which is crucial for understanding pelvic floor disorders and their management. The levator ani muscle is situated at the level of the lower third of the vagina in females and the lower border of the prostate gland in males. This level is essential for evaluating pelvic organ prolapse and urinary incontinence.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the levator ani muscle is not situated at the level of the pelvic brim. The pelvic brim is the boundary between the pelvic cavity and the abdominal cavity, and it is located at a higher level than the levator ani muscle.

**Option B:** This option is incorrect because the levator ani muscle is not situated at the level of the ischial spine. The ischial spine is a bony landmark that marks the lateral boundary of the pelvic cavity, but it is not the correct level for the levator ani muscle.

**Option D:** This option is incorrect because the levator ani muscle is not situated at the level of the pubic symphysis. The pubic symphysis is the cartilaginous joint between the left and right pubic bones, and it is located at a higher level than the levator ani muscle.

**Clinical Pearl / High-Yield Fact**
The levator ani muscle is often referred to as the "sling" of the pelvic floor, providing support to the pelvic organs and maintaining continence. A weakened or damaged levator ani muscle can lead to pelvic organ prolapse and urinary incontinence.

**Correct Answer: C. The lower third of the vagina in females and the lower border of the prostate gland in males.**