Which of the following is a type of point defect in crystalline solids?

• A. Grain boundary
• B. Stacking fault
• C. Dislocation
• D. Vacancy

Answer: (D)

The main idea is distinguishing defect types by how they extend in the crystal. Point defects are zero-dimensional imperfections that disrupt a single lattice site. A vacancy is a missing atom at a lattice site, which exactly fits that zero-dimensional, point-like idea. In contrast, grain boundaries are interfaces between differently oriented grains (a planar defect), stacking faults are irregularities in the stacking sequence of layers (also planar), and dislocations are defects that run along a line through the crystal (a line defect). Since a vacancy is localized to a single lattice site and does not extend along a line or plane, it is the type that defines a point defect.