Calanus is a:

Correct option is A

Correct Answer: (a) Copepod
Explanation:
→ Calanus is a genus of marine copepods (Calanoida) abundant in temperate and polar seas and central to marine food webs.
Information Booster:
→ Large Calanus populations fuel higher trophic levels (fish, whales) in productive seas; their seasonal dynamics influence fish recruitment.
Additional Information (Other Options):
(b) Cladoceran: freshwater group (e.g., Daphnia).
(c) Phytoplankton: plants/algae.
(d) Branchiopod: e.g., fairy shrimps.