### **Official Solution & Concept Analysis (GATE BT 2020 Q.26)**
**1. Governing Concept**:
- **Topic**: Extremophilic Microbiology & Archaea
- Characteristics of extremely halophilic microorganisms isolated from salt-rich environments (e.g., salt lakes, salterns, solar evaporation ponds with
gt; 20\% \text{ NaCl}$):
- Many belong to the archaeal class **Haloarchaea** (e.g., *Halobacterium salinarum*).
- Archaeal cellular hallmarks:
- **Cell membrane lipids contain ether linkages** (glycerol diether / tetraether with branched phytanyl isoprenoid chains), whereas bacterial and eukaryotic membranes feature ester linkages.
- Lack peptidoglycan cell walls (possess protein S-layers).
- Possess ether-linked lipids.
**2. Option Analysis**:
- **Option (A) is INCORRECT**: Extreme halophiles possess ether, not ester linkages.
- **Option (B) is INCORRECT**: Peptidoglycan is bacterial, not archaeal.
- **Option (C) is INCORRECT**: Bacterial flagellin is absent (possess archaeal archaella).
- **Option (D) [ether linked lipids] is CORRECT**: Diagnostic biochemical signature of halophilic Archaea.
**3. Final Answer**:
- **Correct Choice**: **D**