Correct answer: (d) Blue-green algae
Explanation
- A
Red algae
(a) Red algae. Rhodophyta are not the textbook biological nitrogen-fixers.
- B
Brown algae
(b) Brown algae. Phaeophyceae (kelps) do not fix N2 as their defining ecology.
- C
Green algae
(c) Green algae. Chlorophyta are photosynthetic eukaryotes, not the N-fixing group named here.
- D
Blue-green algae
(d) Blue-green algae. Cyanobacteria (Anabaena, Nostoc, Trichodesmium) fix N2 in heterocysts or analogous cells. (d) Official key.
Summary. Official key is (d). Biological nitrogen fixation among the algae options is the cyanobacteria (blue-green algae). Red, brown and green algae are not that guild.
Same topic · past papers
UPPSC has asked this before
These previous-year questions sit on the same topic. Open one to practise the earlier ask.
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