ANS:D - dislocation

The strain hardening effect in metals subjected to cold working is primarily due to the dislocation mechanism. Cold working involves deforming a metal at temperatures below its recrystallization temperature, leading to an increase in dislocation density within the material. This increased dislocation density impedes the movement of dislocations, making it more difficult for them to slip past one another. As a result, the metal becomes harder and stronger.