Grain boundary is a surface imperfection that separates crystals of different orientations in a polycrystalline aggregate.

• A. A region where grains align perfectly.
• B. A surface imperfection which separate crystals of different orientations in a polycrystalline aggregate.
• C. The elastic response of a material to applied stress.
• D. A boundary that prevents dislocation motion entirely.

Answer: (B)

Grain boundaries are interfaces between individual crystals in a polycrystal, where the lattice orientation changes from one grain to the next. This boundary is a defect-rich region that separates grains with different orientations, rather than a perfectly aligned seam. The statement describing a surface imperfection that separates crystals of different orientations in a polycrystalline aggregate captures exactly what a grain boundary is, so it aligns with the physical concept.

Think of how this boundary affects mechanical behavior: it disrupts the regular crystal lattice, acts as a barrier to dislocation motion, and contributes to the strengthening of the material via the Hall–Petch effect. But it’s not a region of perfect alignment, nor is it simply the material’s elastic response to stress. And while grain boundaries impede dislocations, they do not completely stop their motion in all conditions—dislocations can still move or be absorbed at boundaries under certain temperatures and stresses.