**Core Concept**
In prokaryotes, the cell wall and membrane structure are distinct from those found in eukaryotes. The absence of a true nucleus and other membrane-bound organelles in prokaryotes is a key characteristic of their cellular architecture.

**Why the Correct Answer is Right**
The correct answer is related to the unique features of prokaryotic cell membranes. Prokaryotes lack a true nucleus and other membrane-bound organelles, and their cell membrane is not compartmentalized like that of eukaryotes. The presence of a **peptidoglycan layer** (also known as murein) in the cell wall of prokaryotes provides structural support and maintains the cell's shape. This peptidoglycan layer is composed of cross-linked peptides and sugars.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because prokaryotes lack a true nucleus, which is a characteristic feature of eukaryotes.
* **Option B:** This option is incorrect because while some prokaryotes have a capsule, it is not a universal feature of all prokaryotes and is not the correct answer to this question.
* **Option D:** This option is incorrect because while some prokaryotes have flagella, it is not a feature that is present in all prokaryotes.

**Clinical Pearl / High-Yield Fact**
The peptidoglycan layer in prokaryotic cell walls is a key target for beta-lactam antibiotics, such as penicillin and ampicillin, which inhibit cell wall synthesis and ultimately lead to bacterial cell lysis.

**Correct Answer:** C. Peptidoglycan layer