**Core Concept**
The sacrococcygeal ligament is a fibrous structure that connects the sacrum to the coccyx, forming the posterior part of the pelvic floor. Understanding the structures that pass deep to this ligament is essential in pelvic surgery and anatomical studies.

**Why the Correct Answer is Right**
The structures that pass deep to the sacrococcygeal ligament include the gluteus maximus muscle, the sacrotuberous ligament, and the sacrospinous ligament. These structures are involved in the stability and movement of the pelvis. The gluteus maximus muscle is a major muscle of the buttocks that extends and rotates the hip joint. The sacrotuberous and sacrospinous ligaments provide additional support to the pelvic floor and help to stabilize the sacrum.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as it does not provide a specific structure that passes deep to the sacrococcygeal ligament. Without more information, it is difficult to determine what structure this option is referring to.
* **Option B:** This option is incorrect as it is a structure that passes superficial to the sacrococcygeal ligament, not deep to it. The piriformis muscle is a muscle of the buttocks that passes through the greater sciatic foramen and is located superficial to the sacrococcygeal ligament.
* **Option C:** This option is incorrect as it is not a structure that passes deep to the sacrococcygeal ligament. The iliopectineal line is a bony landmark on the ilium that is not directly related to the sacrococcygeal ligament.

**Clinical Pearl / High-Yield Fact**
When performing pelvic surgery, it is essential to identify the sacrococcygeal ligament and the structures that pass deep to it to avoid damage to these critical structures. Familiarity with the anatomy of the pelvic floor is crucial in preventing complications and ensuring successful outcomes.

**Correct Answer:** B. piriformis muscle