The correct answer is A. A tube that extends from a bacterium.
Explanation
Pilus is a term that is used to name the tubes that extend outside the membrane of a bacterium that looks like short hairs anchored to the membrane; these hairs are involved in bacterial conjugation. Sometimes these tubes serve the bacteria to adhere to the organisms in which they will be housed. So, the correct answer is A. A tube that extends from a bacterium.
