**Core Concept**
Insulin secretion from the beta cells of the pancreas is closely regulated and involves the release of several molecules. One of these molecules is C-peptide, which is a byproduct of proinsulin conversion to insulin. C-peptide is secreted in a 1:1 ratio with insulin, making it an important marker for insulin secretion.

**Why the Correct Answer is Right**
C-peptide is a 31-amino acid peptide that is co-secreted with insulin from the beta cells of the pancreas. It is formed during the conversion of proinsulin to insulin, which is the primary function of the enzyme prohormone convertase 2 (PC2). C-peptide is then released into the bloodstream, where its levels can be measured to assess insulin secretion. The 1:1 ratio of C-peptide to insulin is due to the fact that both molecules are derived from the same precursor molecule, proinsulin.

**Why Each Wrong Option is Incorrect**
**Option A:** Pancreatic polypeptide is a hormone secreted by the PP cells of the pancreas, but it is not secreted in a 1:1 ratio with insulin. Instead, it is released in response to nutrient intake and helps to regulate pancreatic enzyme secretion.
**Option B:** Glucagon is a hormone secreted by the alpha cells of the pancreas that raises blood glucose levels by stimulating glycogenolysis and gluconeogenesis. It is not secreted in a 1:1 ratio with insulin and actually acts to counteract insulin's effects on glucose metabolism.
**Option C:** GLP-1 (glucagon-like peptide-1) is an incretin hormone secreted by the L cells of the intestine that enhances insulin secretion and glucose uptake in response to nutrient intake. While GLP-1 does interact with insulin secretion, it is not secreted in a 1:1 ratio with insulin.

**Clinical Pearl / High-Yield Fact**
Measuring C-peptide levels is an important tool for assessing insulin secretion and diagnosing conditions such as type 1 diabetes. In type 1 diabetes, C-peptide levels are typically low due to the autoimmune destruction of beta cells, while in type 2 diabetes, C-peptide levels may be elevated due to insulin resistance.

**✓ Correct Answer: D. C- peptide**