A
They are exclusive marine organisms used to measure oceanic salinity levels.
B
They act as primary apex predators that control large mammalian populations.
C
They are endemic floral species responsible for nitrogen fixation in aquatic soils.
D
They serve as sensitive bio-indicators reflecting the health conditions of freshwater habitats.
Correct answer: (d) They serve as sensitive bio-indicators reflecting the health conditions of freshwater habitats.
Explanation
Option D is correct because odonates are sensitive ecological indicators (bio-indicators) whose status and population reveal the health of freshwater environments. Options A, C, and D are scientifically incorrect as odonates do not control large mammals, fix nitrogen, or live in marine oceanic waters.
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