**Core Concept**
The scapula, or shoulder blade, is a triangular bone that forms the posterior aspect of the shoulder joint. Its position and orientation are crucial for understanding shoulder anatomy and movement. The inferior border of the scapula is a key landmark used to estimate the level of the rib cage.

**Why the Correct Answer is Right**
The inferior border of the scapula lies at the level of the 7th rib. This is due to the anatomical relationship between the scapula and the rib cage. The scapula is positioned such that its inferior border is approximately at the level of the 7th rib, which is a useful reference point for clinicians and anatomists. This relationship is essential for understanding the spatial arrangement of the shoulder joint and the surrounding musculature.

**Why Each Wrong Option is Incorrect**
**Option A:** The 5th rib is too high, and using it as a reference point would result in an inaccurate estimation of the scapular position.
**Option B:** The 6th rib is also too high, and its use would lead to a similar error in estimating the scapular level.
**Option D:** The 8th rib is too low, and its use would result in an overestimation of the scapular position.

**Clinical Pearl / High-Yield Fact**
The inferior border of the scapula can be used as a landmark to estimate the level of the 7th rib, which is crucial for inserting chest tubes, performing thoracocentesis, or identifying the location of the diaphragm.

**✓ Correct Answer: C. 7th**