What best describes a pilus

A tube that extends from a bacterium

Pilus is a hair like tube or appendage that required for bacterium cell in conjugation.

Explanation:

A pilus or pili is a hair-like appendage or tube found on the surface of multiple bacteria and archaea. The term fimbria or pilus are interchangeable but pilus is specially reserved for the appendage needed for bacterial conjugation.

Pili allows for the transfer of DNA between bacteria donor cell to host cell during bacterial conjugation. They are also called sex pili because they permit the exchange of genes via the formation of mating pairs.

Thus, Pilus is a hair like tube or appendage that required for bacterium cell in conjugation.