Which of the following substances is the hardest in the human body?

The hardest substance in the human body is (c) Dentin. Dentin is a calcified tissue that forms the bulk of a tooth, providing structural support and protection. It is composed mainly of hydroxyapatite crystals, which are similar to those found in bone, but dentin is denser and contains less organic material.
While bone (a) is also a hard tissue, dentin is generally considered harder due to its higher mineral content. Dentin makes up the bulk of a tooth’s structure beneath the enamel and cementum layers.
Plaque (b) is a soft, sticky film of bacteria that forms on teeth and can lead to dental decay and gum disease. It is not a hard substance.
Nails (d) are composed of a protein called keratin and are hard, but they are not as hard as dentin or bone. They provide protection to the fingertips but are not as mineralized as dentin.