Plankton living in still fresh water:

Correct option is A

Correct Answer: (a) Heleoplankton
Explanation:
→ Heleoplankton denotes planktonic communities of lentic (still) waters such as lakes and ponds.
→ They include phytoplankton and zooplankton adapted to low-flow environments.
Information Booster:
→ Potamoplankton refers to plankton in lotic (flowing) waters (rivers).
Additional Information (Other Options):
(c) Halioplankton: marine plankton.
(d) Hypalmyroplankton: brackish/freshwater mix zones (rarely used).