ANS:A - low pounhg temperature.

Cold shut defect in casting occurs when two portions of metal flow together during solidification but fail to fuse properly due to insufficient fluidity or inadequate pouring temperature. The correct cause of cold shut defect is low pouring temperature. When the pouring temperature of the molten metal is too low, it solidifies rapidly upon contact with the cooler mold surfaces or previously solidified metal, leading to incomplete fusion between the two streams of metal. This results in a visible seam or line along the casting surface, known as a cold shut defect. The low pouring temperature prevents the molten metal from properly filling the mold cavity and merging together, resulting in the formation of a discontinuity in the casting. Cold shut defects can weaken the casting and compromise its structural integrity, making them undesirable in most casting applications. While other factors such as faulty gating systems, melt-mold reactions, or low mold permeability can contribute to casting defects, they do not directly cause cold shut defects. Therefore, the correct cause of cold shut defect is low pouring temperature.