An exotic weed causing nuisance in Indian waters:

Correct option is D

Correct Answer: (d) Eichhornia crassipes
Explanation:
→ Eichhornia crassipes (water hyacinth) is a notorious invasive free-floating weed introduced to India that forms dense mats, blocking waterways and depleting oxygen.
→ Rapid vegetative propagation makes control difficult.
Information Booster:
→ It impacts navigation, fisheries, irrigation and native biodiversity; control methods include mechanical removal, biological agents and herbicides.
Additional Information (Other Options):
(a) Ipomoea: can be problematic but less widespread.
(b) Nelumbo: native lotus, not an exotic nuisance.
(c) Hydrilla: invasive submerged weed, but Eichhornia is the most notorious floating weed.