Which among the following is a fixed aquatic plant

Fixed (rooted) aquatic plants are anchored in the pond/river bed with leaves that float or emerge. Standard textbook examples include Lotus (Nelumbo nucifera) and Water lily (Nymphaea). Their roots are embedded in mud; long petioles carry leaves/flowers to the surface. 

Do not confuse with freefloating plants like duckweed/water hyacinth (not fixed). If an MCQ lists options such as Lotus, Hydrilla, Water hyacinth, and Wolffia, pick Lotus/Water lily as the fixed plants (Hydrilla is submerged, hyacinth/Wolffia are freefloating).