Answer: Heterocyst.
- A Mesosome
- B Heterocyst
- C Pyrenoid
- D Plasmid
Correct answer: B. Heterocyst
Explanation: Heterocysts are specialized cells in certain filamentous cyanobacteria (e.g., Nostoc, Anabaena) that are the sites of nitrogen fixation.
Whittaker's five-kingdom system (1969) groups all cellular life by cell structure and nutrition; viruses, viroids, and lichens fall outside this system entirely since they aren't independently classifiable as one of the five.
Concept context
Whittaker's five kingdom system covering Monera, Protista, Fungi, plus viruses, viroids, and lichens. A foundational NEET chapter on how living organisms are grouped.