The correct answer is option 3 - Keratin. Nails, hoofs, and horns are made of a protein called keratin. Keratin is a tough protein that forms the structural component of various tissues in animals. It is found in the outermost layer of the skin, hair, feathers, scales, and the hard outer covering of nails, hoofs, and horns.
Cutide, mentioned in option 1, is not a correct term. It might be a typographical error as the correct term is cuticle, which is a waxy layer on the surface of plants or insects.
Chitin, mentioned in option 2, is another type of protein found in the exoskeletons of insects and the cell walls of fungi.
Tunidn, mentioned in option 4, is not a known term and is likely a typographical error.
In conclusion, nails, hoofs, and horns are made of keratin, making option 3 the correct answer.