**Core Concept**
The right costophrenic line is an important landmark in thoracic anatomy, marking the junction between the diaphragm and the rib cage. It is crucial for identifying the position and extent of various thoracic structures, including the liver, spleen, and lungs.

**Why the Correct Answer is Right**
The costophrenic angle is formed by the intersection of the diaphragm and the 10th rib, specifically in the mid-axillary line. This is a standard anatomical reference point, allowing clinicians to accurately locate structures within the thoracic cavity. The diaphragm inserts into the 10th rib at the costal margin, creating this distinctive angle.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately reflect the anatomical position of the costophrenic line. The 8th rib is too high, and the 10th rib is the correct landmark.
**Option B:** This option is incorrect because it suggests a different rib number, which is not supported by anatomical evidence. The costophrenic angle is consistently associated with the 10th rib.
**Option C:** This option is incorrect because it does not provide a specific rib number, making it an ambiguous and unreliable choice.

**Clinical Pearl / High-Yield Fact**
In thoracic anatomy, the costophrenic angle is a critical landmark for identifying the position and extent of various structures within the thoracic cavity. Remembering the 10th rib as the landmark for the costophrenic line will help you accurately locate structures during imaging or surgical procedures.

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