**Core Concept**
Modification of peptide structure of hormones is a crucial process that allows for the regulation of their activity and interaction with target cells. This process involves the conversion of inactive precursor peptides into their active forms, which is essential for the proper functioning of various hormones in the body.

**Why the Correct Answer is Right**
The modification of peptide structure of hormones typically occurs in the endoplasmic reticulum (ER) or the Golgi apparatus of cells. This process involves the action of various enzymes, including prohormone convertases (PCs) and carboxypeptidases. For example, in the case of proinsulin, the conversion of the precursor peptide to insulin involves the removal of a C-peptide fragment by carboxypeptidase B. This process is essential for the proper functioning of insulin, which plays a critical role in glucose metabolism.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because it does not specify a cellular location where hormone modification occurs.

**Option B:**
This option is incorrect because it does not accurately describe the process of hormone modification. While the Golgi apparatus is involved in protein modification, the specific process of converting precursor peptides to active hormones is not limited to this organelle.

**Option C:**
This option is incorrect because it does not accurately describe the cellular location where hormone modification occurs. The ER is indeed involved in protein synthesis and modification, but the specific process of converting precursor peptides to active hormones is not solely limited to this organelle.

**Clinical Pearl / High-Yield Fact**
It's worth noting that defects in the process of hormone modification can lead to various endocrine disorders, such as insulin resistance in diabetes mellitus. Understanding the process of hormone modification is essential for the diagnosis and treatment of these disorders.

**Correct Answer:** D.