These are hollow, non-helical, filamentous appendages:

Explanation

The correct answer is Pili. Pili are hollow, non-helical, filamentous structures found on the surface of many bacteria. They play critical roles in bacterial conjugation, a process of genetic exchange, and in the attachment of bacteria to different surfaces, which can be crucial for colonization and infection. Unlike flagella, which are helical and involved in motility, pili do not contribute to bacterial movement. Trichomes and plasmids are unrelated to the description provided, as trichomes are not hollow, non-helical bacterial appendages, and plasmids are DNA molecules, not structural appendages.