Which of the following is associated with biological nitrogen fixation?

The correct answer is D (Blue-green algae), which are cyanobacteria. They can convert atmospheric nitrogen (N2) into ammonia, which is biological nitrogen fixation. Filamentous forms such as Anabaena and Nostoc do this in specialised cells called heterocysts, which contain the enzyme nitrogenase and keep oxygen away from it. The Azolla-Anabaena association is the classic example, long used as green manure in paddy fields.

Why the others are wrong:

• Red algae (A) and brown algae (B) are marine eukaryotes with no known nitrogen-fixing role relevant to this question.
• Green algae (C) are eukaryotes. A few reports of nitrogen fixation exist under special conditions, but they are not the group associated with this concept in standard texts.

Core concept: nitrogen fixation by prokaryotes, both free-living and symbiotic. It links to Rhizobium in legumes and to biofertiliser programmes, which is a useful Mains angle on soil health and fertiliser use.

Trap: candidates pick green algae or assume all algae fix nitrogen. The nitrogen-fixing algae here are cyanobacteria, which are prokaryotes, not true algae in the strict sense.