Which of the following is an example of a structural protein?

Keratin is classified as a structural protein due to its significant role in forming the structural components of various biological tissues. This fibrous protein is a key component of the skin, hair, and nails, providing mechanical strength and protection. In these tissues, keratin helps maintain integrity and resilience, making it essential for cellular structure and function.

Other proteins listed serve different functions: Myosin is primarily a motor protein involved in muscle contraction; insulin is a hormone that regulates glucose metabolism; and thrombin is an enzyme that plays a crucial role in blood clotting. Each of these proteins has specialized roles that do not pertain to providing structural support, unlike keratin. Thus, keratin stands out as the clear example of a structural protein in this context.