**Core Concept**
The peptidoglycan layer, also known as murein, is a critical component of bacterial cell walls, providing structural integrity and maintaining the osmotic balance between the cell and its environment. The thickness of the peptidoglycan layer varies among different bacterial species, influencing their susceptibility to antibiotics and environmental stressors.

**Why the Correct Answer is Right**
In gram-positive bacteria, the peptidoglycan layer is thicker and more cross-linked compared to gram-negative bacteria. This increased thickness provides greater structural stability and resistance to osmotic shock. The thick peptidoglycan layer in gram-positive bacteria also makes them more susceptible to beta-lactam antibiotics, such as penicillin, which target the bacterial cell wall synthesis machinery.

**Why Each Wrong Option is Incorrect**
* **Option A:** Gram-negative bacteria have a thinner peptidoglycan layer, typically consisting of 6-7 glycan chains, whereas gram-positive bacteria have a thicker layer with 20-30 glycan chains.
* **Option B:** Mycoplasma, a type of bacteria, lacks a peptidoglycan layer altogether, instead having a cell membrane that maintains its structural integrity.
* **Option D:** Archaea, a distinct domain of prokaryotes, have a different type of cell wall, known as a S-layer, which is composed of protein or glycoprotein layers.

**Clinical Pearl / High-Yield Fact**
The thickness of the peptidoglycan layer is a key factor in determining the susceptibility of bacteria to beta-lactam antibiotics. Gram-positive bacteria, with their thicker peptidoglycan layers, are generally more susceptible to these antibiotics than gram-negative bacteria.

**Correct Answer: C. Gram-positive bacteria.**