The correct answer is option 2, enamel. Enamel is the hardest substance in the human body. It is found on the outer layer of the teeth and acts as a protective covering. Enamel is mainly composed of minerals, primarily hydroxyapatite, which makes it extremely strong and durable. It is even stronger than bone, which is the next hardest substance in the body. Bone, option 1, is a strong and rigid connective tissue, but it is not as hard as enamel. Dentine, option 3, is a calcified tissue that forms the majority of the tooth structure, but it is also not as hard as enamel. Nail, option 4, is made of keratin, a protein, and it is much softer than enamel. Therefore, the hardest substance in the human body is enamel.