Explanation:

The B – H curve, also known as the magnetization curve, is a graphical representation of the relationship between the magnetic field strength (H) and the magnetic flux density (B) in a magnetic material. It helps to understand the magnetic properties of a material.

The B – H curve for different materials can have different shapes. While some materials exhibit a linear relationship between B and H, others show nonlinear behavior. In this case, the correct option is (d) Soft iron, which does not have a straight line B – H curve.

Soft Iron:
Soft iron is a ferromagnetic material that is commonly used in electromagnets and magnetic circuits due to its high magnetic permeability. Soft iron has a nonlinear B – H curve due to its high saturation magnetization.

Bullet Points:
- Soft iron is a ferromagnetic material.
- It has a high magnetic permeability.
- The B – H curve for soft iron is nonlinear.
- Soft iron exhibits high saturation magnetization.

Other Options:
Let's briefly discuss the B – H curves for the other options mentioned in the question:

Copper:
Copper is a non-magnetic material and does not exhibit any significant magnetic properties. It does not have a B – H curve as it does not respond to magnetic fields.

Silicon steel:
Silicon steel is a ferromagnetic material that is commonly used in the construction of electrical transformers and motors. The B – H curve for silicon steel is nonlinear but relatively straight compared to soft iron. It exhibits a higher saturation magnetization compared to soft iron.

Air:
Air is a non-magnetic material and does not exhibit any significant magnetic properties. It does not have a B – H curve as it does not respond to magnetic fields.

Therefore, the correct option is (d) Soft iron, which has a nonlinear B – H curve due to its high saturation magnetization.