A magnetising force of 800 AT/m will produce a flux density of ________in air.

Correct option is A

​$$\text{The relation between } B \text{ and } H \text{ is:} \\B = \mu H \\\text{Where,} \\\mu = \mu_0 \mu_r \\\bullet \ \mu = \text{Permeability of the material} \\\bullet \ \mu_0 = \text{Absolute permeability} = 4\pi \times 10^{-7} \, \text{H/m} \\\bullet \ \mu_r = \text{Relative permeability} \\\textbf{Calculation:} \\\text{Given,} \\H = 800 \, \text{AT/m}, \quad \mu_r = 1 \, (\text{since it's for air}) \\\text{So, the flux density for the given magnetizing force will be:} \\B = 4\pi \times 10^{-7} \times 800 = 1.0053 \times 10^{-3} \\B \approx 1 \, \text{mWb/m}^2$$​​