**Core Concept**
The question is testing the student's knowledge of the anatomical relationship between the rectum and surrounding structures in the pelvis. The rectum is a part of the large intestine, and its posterior relationship is crucial in understanding pelvic anatomy and potential complications during surgeries or medical procedures.

**Why the Correct Answer is Right**
The rectum is surrounded by several structures posteriorly, including the sacrum, coccyx, and the sacrorectal ligaments. The sacrum is a triangular bone located at the base of the spine, and it forms the posterior wall of the pelvis. The coccyx, or tailbone, is a small, triangular bone located at the very bottom of the spine. The sacrorectal ligaments are fibrous bands that connect the sacrum to the rectum, providing support and stability to the pelvic floor.

**Why Each Wrong Option is Incorrect**
**Option A:** The sacrum is indeed in a posterior relationship to the rectum, so this option is incorrect.
**Option B:** The coccyx is also in a posterior relationship to the rectum, making this option incorrect as well.
**Option C:** This option is correct because the rectal ampulla is the part of the rectum that is anterior to the sacrum and coccyx, not posterior.

**Clinical Pearl / High-Yield Fact**
In pelvic surgeries, the rectum is often at risk of injury due to its close proximity to surrounding structures. A thorough understanding of pelvic anatomy is crucial in preventing such complications.

**Correct Answer: C.**