To solve this problem, we need to understand the structures and functions of different bacterial appendages.$\newline$Bacteria can have several types of appendages, each serving different purposes:$\newline$• Fimbriae: These are short, hair-like structures that help bacteria adhere to surfaces or other cells. They are not involved in motility.$\newline$• Flagella: These are long, whip-like structures that rotate to propel the bacterium forward. They are the primary structures responsible for bacterial motility.$\newline$• Cilia: These are short, hair-like structures found in eukaryotic cells, not bacteria. They are not involved in bacterial motility.$\newline$• Pili: These are hair-like structures that are generally involved in the exchange of genetic material during conjugation, not in motility.$\newline$Given this information, the correct answer is that motile bacteria move by using flagella.$\newline$Therefore, the correct option is Option 2: flagella.