**Core Concept**
C-peptide is a peptide fragment that serves as a marker for insulin production in the body. It is a byproduct of proinsulin, a precursor to insulin, and its levels can be used to measure insulin production in patients with diabetes mellitus.

**Why the Correct Answer is Right**
C-peptide is specifically produced during the processing of proinsulin to insulin in the pancreas. As a result, its levels are directly proportional to insulin production. Elevated levels of C-peptide in the blood indicate endogenous insulin production, whereas low levels suggest exogenous insulin therapy or pancreatic insufficiency.

**Why Each Wrong Option is Incorrect**
**Option A:** Not applicable (no relevant information provided).
**Option B:** Incorrect because C-peptide is not a marker for glucagon production.
**Option C:** Incorrect because C-peptide is not a marker for growth hormone production.

**Clinical Pearl / High-Yield Fact**
C-peptide levels can be used to differentiate between type 1 and type 2 diabetes mellitus, with type 1 diabetes typically associated with low C-peptide levels due to pancreatic destruction. This is an important clinical consideration when managing patients with diabetes.

**Correct Answer: C. C-peptide is a marker for endogenous insulin production.**