ANS:D - low stacking fault energy.

Dislocation cross-slip is difficult in materials with low stacking fault energy. Dislocation cross-slip is a mechanism by which dislocations change their glide plane during plastic deformation. It involves the movement of dislocations from one slip plane to another, which can enhance the material's ductility and deformation behavior. Materials with low stacking fault energy have fewer energy barriers for dislocations to overcome when they cross-slip. As a result, they are more likely to exhibit cross-slip behavior, which contributes to enhanced ductility and deformation mechanisms. On the other hand, materials with high stacking fault energy have higher energy barriers for dislocations to cross-slip, making this mechanism more difficult. In such materials, dislocations tend to remain confined to their original slip planes, leading to increased work hardening and potentially limiting ductility. Therefore, materials with low stacking fault energy are less resistant to dislocation cross-slip, making it easier for dislocations to undergo this mechanism during plastic deformation.