Which of the following is characterized by hairlike projections that extend from the plasma membrane and assist in locomotion?

The presence of hairlike projections extending from the plasma membrane that assist in locomotion is a defining characteristic of cilia. Cilia are short, numerous appendages found on the surface of many eukaryotic cells. They have a coordinated beating motion that allows for movement, either the movement of the cell itself or the movement of fluids and particles across the cell surface. Each cilium is made up of microtubules arranged in a specific "9+2" structure that facilitates this movement.

In contrast, microvilli are short and non-motile extensions of the plasma membrane that primarily function to increase surface area for absorption and do not assist in locomotion. Pseudopodia, often observed in amoeba, are temporary projections that allow for movement and feeding but are not hairlike and are more irregular in shape. Flagella, while also involved in locomotion, are typically longer and fewer in number compared to cilia, and they have a different structure and mechanism of movement. Thus, cilia is the most fitting answer for the characteristics described in the question.