The correct answer is: C. 1.3 Wb/m2
The flux density is the magnetic field strength per unit area. It is a measure of how much magnetic flux is concentrated in a given area. The higher the flux density, the stronger the magnetic field.
The flux density in a transformer is limited by the saturation of the core material. When the core material is saturated, it can no longer increase the magnetic field strength. This can lead to a loss of efficiency and overheating of the transformer.
The flux density in a 275 kV transformer using cold rolled grain oriented steel is typically limited to 1.3 Wb/m2. This is because cold rolled grain oriented steel has a high saturation magnetization, which means that it can be magnetized to a high level before it saturates.
The other options are incorrect because they are either too low or too high. A flux density of 1.0 Wb/m2 is too low and would not result in a strong enough magnetic field. A flux density of 1.6 Wb/m2 is too high and would cause the core material to saturate.