ANS:D - prevent cold cracks.

Preheating before welding is done primarily to prevent cold cracks. Cold cracks, also known as hydrogen-induced cracking or delayed cracking, can occur in the heat-affected zone (HAZ) of a weldment due to the presence of hydrogen and residual stresses. Preheating helps reduce the cooling rate of the weld metal and the surrounding base metal, which minimizes the risk of hydrogen becoming trapped in the weld and the HAZ. By reducing the risk of hydrogen embrittlement, preheating helps prevent the formation of cold cracks and improves the overall integrity of the weld joint. While preheating can also help reduce plate distortion by minimizing thermal stresses and can aid in burning off oil, grease, and other contaminants on the plate surface, its primary purpose is to prevent cold cracks.