Answer:
b. point defect in which an atom is missing from a lattice site in the crystal.
Explanation:
A vacancy is a point defect in which atom is missing in crystal and because of this irregular crystal lattice is formed.
Point defects are of different types:
Self interstitial
Substitution impurity
Interstitial impurity
Vacancy
It is the point defect in which an extra atom occupy interstitial void. They are occur in very low concentration.
It is the type of defect in which another atom comes and occupy the position of atom of bulk. The size of impurity atom is approximately equal to the bulk atom.
In interstitial impurity a smaller atoms occupy the space in the lattice not in the place of atoms of bulk. It means smaller atom fit into the vacant space between the atoms.
It is the type of defect in which atom is missing. It means vacancy is created in place of atom. It is mostly occur at temperature when kinetic energy increased and atoms escape from its position and randomly move thus changed their position and create empty space.