ANS:D - Cast iron

Out of the options provided, cast iron will fracture most readily when hit with a hard hammer. Cast iron is relatively brittle compared to brass, bronze, and German silver. When subjected to a forceful impact, such as being hit with a hard hammer, cast iron is more prone to fracture due to its brittle nature. It tends to shatter or break apart rather than deform or absorb the impact like the other materials listed. Brass, bronze, and German silver are all alloys that typically exhibit greater ductility and toughness compared to cast iron, making them less prone to fracture when subjected to impact forces. They are more likely to deform or dent rather than shatter under the same conditions.