The area of the hysteresis loop is a measure of energy loss in a magnetic material when it is subjected to a changing magnetic field. A smaller area represents lower energy loss and thus, a more efficient material. In this case, the correct answer is option 'B' - silicon steel.

Explanation:
1. Hysteresis Loop:
- The hysteresis loop is a graphical representation of the relationship between the magnetic field strength (H) and the magnetic flux density (B) in a material.
- It shows the magnetization characteristics of a material when it is subjected to an alternating magnetic field.
- The loop is formed when the magnetic field is gradually increased and then decreased, forming a closed curve.

2. Area of the Hysteresis Loop:
- The area enclosed by the hysteresis loop represents the energy loss per cycle in the material.
- It is directly proportional to the energy dissipated as heat during each cycle of magnetization and demagnetization.

3. Comparison of Materials:
- Wrought iron, hard steel, soft iron, and silicon steel are all commonly used magnetic materials.
- However, they have different magnetic properties that affect the shape and size of their hysteresis loops.

4. Wrought Iron:
- Wrought iron has a large hysteresis loop with a relatively high coercivity and magnetic retentivity.
- It requires a significant amount of energy to magnetize and demagnetize, resulting in a larger area of the hysteresis loop.
- Therefore, wrought iron will have a larger energy loss and is not the material with the least area.

5. Hard Steel:
- Hard steel has a smaller hysteresis loop compared to wrought iron.
- It has higher coercivity and magnetic retentivity than soft iron, but lower than wrought iron.
- The smaller loop indicates lower energy loss and higher efficiency.
- However, hard steel is not the material with the least area.

6. Soft Iron:
- Soft iron has a much smaller hysteresis loop compared to both wrought iron and hard steel.
- It has low coercivity and magnetic retentivity, making it easier to magnetize and demagnetize.
- The smaller loop represents lower energy loss and higher efficiency compared to both wrought iron and hard steel.
- However, soft iron is not the material with the least area.

7. Silicon Steel:
- Silicon steel is a ferromagnetic material that is specifically designed to have low hysteresis loss.
- It has high magnetic permeability, low coercivity, and low magnetic retentivity.
- Silicon steel exhibits a hysteresis loop with the smallest area among the given options.
- The smaller loop represents lower energy loss and higher efficiency compared to all the other materials.
- Therefore, silicon steel is the material with the least area of the hysteresis loop.

In conclusion, the area of the hysteresis loop will be least for silicon steel among the given options of wrought iron, silicon steel, hard steel, and soft iron.