In drop forging, the forging process involves shaping a workpiece by the repeated striking of a hammer against a die. The correct answer is option 'C', which states that forging is done by dropping the die with the hammer at high velocity. Let's understand this answer in detail.

1. Introduction to drop forging:
- Drop forging is a metalworking process used to shape heated metal stock into desired shapes.
- It is a type of forging method where a hammer is used to strike the workpiece onto a die, which results in the required shape.

2. The purpose of drop forging:
- Drop forging is primarily used to produce parts that require high strength, durability, and accuracy.
- It is commonly used in the automotive, aerospace, and construction industries to manufacture components like crankshafts, connecting rods, gears, and axles.

3. Explanation of option 'A' - The workpiece at high velocity:
- This option is incorrect because the workpiece is not dropped at high velocity in drop forging.
- The workpiece is heated to a specific temperature and then placed on the die.
- The hammer, along with the die, is dropped at high velocity onto the workpiece to shape it.

4. Explanation of option 'B' - The hammer at high velocity:
- This option is incorrect because the hammer alone is not dropped at high velocity in drop forging.
- The hammer is used in conjunction with the die and is dropped as a unit onto the workpiece.
- The high velocity of the hammer's impact generates the necessary force to shape the workpiece.

5. Explanation of option 'C' - The die with hammer at high velocity:
- This option is correct because drop forging involves dropping the die with the hammer at high velocity.
- The die is positioned above the workpiece, and the hammer, attached to the die, is lifted to a certain height.
- The hammer is then released, and gravity causes it to fall rapidly, impacting the die and exerting a force on the workpiece.

6. Explanation of option 'D' - A weight on the hammer to produce the requisite impact:
- This option is incorrect because drop forging does not involve placing a weight on the hammer.
- The hammer itself is designed to have enough weight to generate the required impact force.
- The weight and velocity of the falling hammer are carefully controlled to achieve the desired shaping effect on the workpiece.

In conclusion, drop forging involves dropping the die with the hammer at high velocity onto the workpiece. This method allows for the precise shaping of heated metal stock into strong and durable components.