Keratin, the protein that strengthens hair and nails, belongs to which group of cytoskeletal fibers?

1. The cytoskeleton has three fiber types: microfilaments, intermediate filaments, and microtubules. 2. Intermediate filaments are made of several strands of fibrous proteins wound together. 3. Keratin, familiar from hair, nails, and the epidermis of the skin, is one of those fibrous proteins. 4. Microfilaments are built from actin, so the actin option and the microfilament option name the same wrong fiber twice. 5. Microtubules are hollow tubes of tubulin that partner with cilia and flagella instead. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 4 "Cell Structure", section 4.5 The Cytoskeleton_