A student observes an organism with a peptidoglycan cell wall, no membrane-bound nucleus, and the ability to fix nitrogen using heterocysts. This organism most likely belongs to which group?

Correct answer: B. Cyanobacteria (Eubacteria)

Explanation: Cyanobacteria (blue-green algae), classified under eubacteria within Monera, are photosynthetic prokaryotes with chlorophyll a and a rigid cell wall; some filamentous forms like Nostoc and Anabaena fix nitrogen using heterocysts.