Final answer:
The scientist is likely observing a ciliate, a type of protozoa with hairlike projections called cilia, using a light microscope enhanced with special stains to make the structures visible.
Step-by-step explanation:
If a scientist is observing a freshwater sample under a light microscope and notices a moving cell with small, hairlike projections, the cell is most likely a ciliate or some type of protozoa that uses cilia for movement and feeding. These microscopic organisms are a common sight in freshwater environments, and their distinctive cilia are used for locomotion and to sweep food particles towards the cell's mouth, which is known as the oral groove. For a clear observation, scientists often use special stains that bind to different components of cells and enhance contrast, since the components are otherwise not very distinguishable due to the transparency of living cells. However, staining generally kills the cells, which is acceptable for studying cell structure but not ideal for observing living processes.