Final answer:
Intermediate filaments are the toughest and most durable of the three types of cytoskeletal filaments. Keratin, a fibrous protein found in intermediate filaments, strengthens hair, nails, and skin.
Step-by-step explanation:
The toughest and most durable of the three types of cytoskeletal filaments is intermediate filaments. For example, keratin, a fibrous protein found in intermediate filaments, strengthens hair, nails, and skin.