a. Azolla

b. Wolffia

c. Eichhornia

d. Trapa

Answer: c. Eichhornia

Explanation: Eichhornia crassipes (water hyacinth) is the world’s most serious aquatic weed, also known as the “Terror of Bengal”. Because of its high size, it can cover the surface of all water bodies, compress it, and pose a threat to other marine life.