The answer is: c) Flagella.
Explanation: Flagella are whip-like structures that enable motility in many types of cells, including sperm cells. They move in a coordinated manner, allowing cells to swim through liquid environments. Cilia, while also capable of movement, typically create a wave-like motion to move fluid or particles over the cell surface rather than the cells themselves. Conversely, microvilli are not motile; their primary function is to increase surface area for absorption, particularly in epithelial cells, such as those in the intestines. Thus, among the given options, it is flagella that are always motile.