The hardest part of the body of a vertebrate animal is

The hardest part of the body of a vertebrate animal is (c) Enamel. Enamel is the outermost layer of teeth and is primarily composed of a mineralized crystalline calcium phosphate known as hydroxyapatite. This unique composition gives enamel its exceptional hardness and resistance to wear and tear.
Enamel serves a vital role in protecting the underlying dentin and pulp of the tooth from mechanical stress, bacteria, and acids present in the oral environment. It also provides the necessary strength for biting and chewing food.
While bone (b) is a hard tissue that provides structural support and protection to various organs and tissues in the body, enamel is generally considered to be harder due to its highly mineralized composition.
Keratin (a) is a protein found in structures such as hair, nails, and the outer layer of the skin. While keratin provides toughness and strength to these tissues, it is not as hard as enamel.
The skull (d) is composed of various bones, including the cranium and facial bones, which protect the brain and sensory organs. While the skull is strong and durable, its individual bones are not as hard as enamel.