**Core Concept**
The right costo-phrenic line is an anatomical landmark on chest radiographs, representing the intersection of the diaphragm and the rib cage. It is used to assess the position and shape of the diaphragm. The level at which this line intersects the mid-axillary line is important for diagnosing conditions such as pneumothorax or diaphragmatic elevation.

**Why the Correct Answer is Right**
The normal position of the right costo-phrenic angle is at the level of the 10th rib in the mid-axillary line. This is because the diaphragm typically rises to this level during expiration. The right hemidiaphragm is usually slightly higher than the left due to the presence of the liver.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it suggests a level that is too high for the normal position of the right costo-phrenic angle.
**Option B:** Similarly, this is too low and does not accurately reflect the typical anatomical position.
**Option D:** This option is also incorrect as it does not correspond to the standard reference point for the right costo-phrenic angle.

**Clinical Pearl / High-Yield Fact**
Remembering that the right costo-phrenic angle typically reaches the 10th rib in the mid-axillary line is crucial for identifying abnormalities on chest X-rays, such as blunting of this angle which may indicate pleural effusion.

**Correct Answer:** C. 10th rib