**Core Concept**
The question is testing the understanding of the processing and expression of apolipoproteins, specifically apo B48 and apo B100, which are crucial for lipid metabolism and transport in the body. These apolipoproteins are essential components of chylomicrons and low-density lipoproteins (LDL), respectively.

**Why the Correct Answer is Right**
Apo B48 and apo B100 are expressed as two different apoproteins due to a difference in the length of the primary transcript. Apo B48 is synthesized in the small intestine and is produced through the action of the enzyme microsomal triglyceride transfer protein (MTTP). MTTP cleaves the primary transcript of apo B at a specific site, resulting in the formation of apo B48. In contrast, apo B100 is synthesized in the liver and is produced through the translation of the full-length primary transcript. The difference in the length of the primary transcript leads to the expression of two distinct apoproteins with different functions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the reason for the difference in apo B48 and apo B100 expression. While the translation efficiency of apo B48 and apo B100 may vary, this is not the primary reason for their expression as two distinct apoproteins.

**Option B:** This option is incorrect because it implies that the difference in apo B48 and apo B100 expression is due to a difference in their secondary structure. However, the difference in the length of the primary transcript is the primary reason for their expression as two distinct apoproteins.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that apo B48 and apo B100 have different functions in lipid metabolism. Apo B48 is involved in the transport of dietary lipids from the small intestine to the liver, while apo B100 is involved in the transport of LDL cholesterol from the liver to peripheral tissues.

**Correct Answer:** C. Difference in the length of the primary transcript.