Which of the following are known as “wheel animalcules”?

Correct option is B

Correct Answer: (b) Rotifers
Explanation:
→ Rotifers are nicknamed “wheel animalcules” because of the ciliated anterior corona that appears like a rotating wheel when beating — used for feeding and locomotion.
Information Booster:
→ Rotifers are important micro-zooplankton in freshwater and brackish systems and are widely used as live feed for fish larvae.
Additional Information (Other Options):
(a) Plankton: broad category.
(c) Protozoa: single-celled organisms (some planktonic).
(d) Tubifex: oligochaete worm, benthic.