Muramic acid is an amino sugar acid. In terms of chemical composition, it is the ether of lactic acid and glucosamine. It occurs naturally as N-acetylmuramic acid in peptidoglycan, whose primary function is a structural component of many typical bacterial cell walls.

**Introduction:**

Muramic acid is a sugar derivative that is found in the cell walls of bacteria. It is a key component of the peptidoglycan layer, which provides structural support to the bacterial cell wall. Muramic acid is not found in the cell walls of other organisms such as green algae, yeast, or fungi.

**Explanation:**

**1. Bacteria:**

- Bacteria are prokaryotic organisms that have a distinct cell wall structure.
- The cell wall of bacteria is composed of peptidoglycan, which is a polymer made up of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) residues.
- Muramic acid is derived from N-acetylmuramic acid and is an essential component of peptidoglycan.
- The presence of muramic acid in the cell wall gives bacteria their characteristic shape and provides structural support.

**2. Green algae:**

- Green algae are eukaryotic organisms that belong to the plant kingdom.
- Unlike bacteria, green algae have a cell wall composed of cellulose, hemicellulose, and pectin, but not peptidoglycan.
- Therefore, muramic acid is not present in the cell walls of green algae.

**3. Yeast:**

- Yeast are eukaryotic microorganisms that belong to the fungal kingdom.
- The cell wall of yeast is primarily composed of glucans and mannoproteins, which differ in composition from bacterial peptidoglycan.
- Hence, muramic acid is not present in the cell walls of yeast.

**4. Fungi:**

- Fungi are eukaryotic organisms that include a wide range of species such as molds, mushrooms, and yeasts.
- The cell wall of fungi is primarily composed of chitin, glucans, and mannoproteins.
- Chitin is a polysaccharide that is structurally similar to cellulose but contains nitrogen.
- Muramic acid is not found in the cell walls of fungi, as they do not contain peptidoglycan.

Therefore, the correct answer is option 'A' - Muramic acid is present in the cell walls of bacteria. It is a key component of the peptidoglycan layer, which provides structural support to bacterial cells.

Muramic acid is an amino sugar acid. In terms of chemical composition, it is the ether of lactic acid and glucosamine. It occurs naturally as N-acetylmuramic acid in peptidoglycan, whose primary function is a structural component of many typical bacterial cell walls.