The correct answer is option (D) By causing movement of the organism.
The unicellular organisms use cilia, flagella or a pseudopod for the purpose of movement and locomotion in aquatic environment. It helps the organism to move from one place to another thereby also escaping from the predators and contributing in its survival. They create water currents in the aquatic environment and move from from place to another in a water body. Flagella and cilia prduce swimming movement and pseudopods produce sliding or amoeboid movements in water. Though flagella and cilia are involved in sensory actions, pseudopods are typically involved in movement and capturing the food.