Final answer:
The hard protective plate made of keratin, also known as onyx, is the nail plate. It covers and protects the fingertips and toes.
Step-by-step explanation:
The hard protective plate composed mainly of keratin, the fiber-shaped protein found in skin and hair, is known as the nail plate. Keratin is a tough, fibrous protein that gives skin, hair, and nails their hard, water-resistant properties. This protein is produced by keratinocytes, which are the predominant cell type found in the epidermis. Nails provide a protective cover over the fingertips and toes, and they are composed mainly of keratin. Therefore, the correct answer to the question is D) Nail plate.