B-H curve is a graph that represents the relationship between the magnetic field strength (H) and the magnetic flux density (B) of a material. It is commonly used to characterize the magnetic properties of different materials. In this case, we are asked to determine the shape of the B-H curve for a specific material.

The correct answer is option 'A', which is air. The B-H curve for air will be a straight line passing through the origin.

Here is the explanation:

1. B-H Curve:
The B-H curve is used to describe the magnetic behavior of a material. It illustrates how the magnetic flux density (B) changes with respect to the magnetic field strength (H). The curve is obtained by measuring the magnetic properties of the material under varying magnetic field strengths.

2. Shape of B-H Curve:
The shape of the B-H curve depends on the material being tested. Different materials exhibit different magnetic properties, and therefore, have different B-H curves. The shape can be classified into different types, such as linear, non-linear, or hysteresis loop.

3. B-H Curve for Air:
Air is a non-magnetic material, which means it does not possess any inherent magnetic properties. It does not exhibit any significant magnetic behavior when subjected to a magnetic field. Therefore, the B-H curve for air is a straight line passing through the origin.

4. Explanation of a Straight Line Curve:
A straight line passing through the origin indicates that there is a linear relationship between the magnetic field strength (H) and the magnetic flux density (B). This means that the magnetic flux density is directly proportional to the magnetic field strength for air.

5. Why is the B-H Curve for Air a Straight Line?
Air is a paramagnetic material, which means it only exhibits a weak magnetic response when exposed to a magnetic field. The magnetic susceptibility of air is very close to zero, thus resulting in a linear relationship between H and B. This linear relationship indicates that the magnetic flux density in air increases proportionally with the magnetic field strength.

In conclusion, the B-H curve for air is a straight line passing through the origin because air is a non-magnetic material with a magnetic susceptibility close to zero. This indicates a linear relationship between the magnetic field strength and the magnetic flux density for air.