**Core Concept**
The coeliac plexus is a complex network of nerve fibers located in the abdominal cavity, playing a crucial role in the autonomic nervous system's control over abdominal organs. It is formed by the convergence of preganglionic sympathetic fibers from the thoracic and lumbar spinal cord.

**Why the Correct Answer is Right**
The coeliac plexus is situated in the abdominal cavity, specifically at the level of the first lumbar vertebra (L1) and the aortic hiatus of the diaphragm. It surrounds the abdominal aorta, the celiac trunk, and the superior mesenteric artery. The coeliac plexus receives preganglionic sympathetic fibers from the thoracic (T5-T9) and lumbar (L1) spinal cord segments. These fibers synapse with postganglionic neurons within the plexus, which then project to various abdominal organs, including the stomach, small intestine, liver, and pancreas.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the coeliac plexus is not located in the thoracic cavity, where the lungs and heart are situated.
**Option B:** This option is incorrect because the coeliac plexus is not located at the level of the pelvic viscera, where the reproductive organs and lower gastrointestinal tract are found.
**Option C:** This option is incorrect because the coeliac plexus does not extend to the neck, where the carotid plexus and superior cervical ganglion are located.

**Clinical Pearl / High-Yield Fact**
The coeliac plexus is an important landmark for abdominal surgeries, particularly those involving the aorta, celiac trunk, and superior mesenteric artery. Damage to the coeliac plexus can lead to abdominal pain, diarrhea, and other autonomic dysfunction symptoms.

**Correct Answer:** C. At the level of the first lumbar vertebra (L1) and the aortic hiatus of the diaphragm.