The correct answer is option 'D', which states that the range of horsepower of electric motor drives for rolling mills is 100 to 500 HP. Let's understand why this is the correct answer.

Electric motor drives are used in rolling mills to provide the necessary power to drive the rolling process. Rolling mills are industrial machines used in the metalworking industry to shape metal products by passing them between pairs of rolls. These rolls apply force to the metal, causing it to deform and change shape.

Here's a breakdown of the different options and why option 'D' is the correct range:

1. Option 'A' - 1 to 10 HP:
- This range of horsepower is too low for rolling mills. Rolling mills require a significant amount of power to exert the necessary force on the metal and overcome the resistance to deformation. 1 to 10 HP is typically insufficient for this purpose.

2. Option 'B' - 15 to 25 HP:
- Similar to option 'A', this range of horsepower is still too low for rolling mills. While it is slightly higher than option 'A', it is still not enough to provide the required power for efficient rolling mill operations.

3. Option 'C' - 50 to 100 HP:
- This range of horsepower is closer to the power requirements of rolling mills. However, it may still be insufficient for larger rolling mills or mills processing harder metals. While this range could be suitable for smaller rolling mills or mills processing softer metals, it does not cover the full range of rolling mill applications.

4. Option 'D' - 100 to 500 HP:
- This range of horsepower is the correct answer. It covers a wider range of rolling mill applications, including both smaller and larger mills. The higher end of the range, 500 HP, can provide the necessary power for larger mills or mills processing harder metals, while the lower end of the range, 100 HP, can be suitable for smaller mills or mills processing softer metals.

In conclusion, the range of horsepower of electric motor drives for rolling mills is generally 100 to 500 HP, as option 'D' correctly states. This range covers a wide variety of rolling mill applications and can provide the necessary power for efficient and effective rolling operations.