Answer: A. decreases with increasing cross-sectional area of material
The magnetic reluctance of material decreases with the increasing cross-sectional area of the material.
Magnetic reluctance is a term related to magnetic circuits, and magnetic reluctance is a property of magnetic material; it is defined as the opposition of a magnetic circuit to the flow(or) to produce magnetic flux through it. Magnetic reluctance is analogous to resistance in electrical circuits.
Just like a resistor which opposes the flow of current magnetic, reluctance will also offer opposition to magnetic flux. When the resistor opposes the current flow, there will be energy dissipation in the electric circuit, but magnetic reluctance does not offer energy dissipation; instead, it stores energy in the magnetic circuit.
Mathematically, reluctance can be expressed as R=l/(μ0.μr.A)
Here, l is the length of the magnetic circuit in meters,
μ0= permeability of vacuum =4π×10-7 H/m
μr= relative permeability of the material
A= area of cross-section of the circuit (meter2)
So from the above equation, we can conclude that the magnetic reluctance of a material is inversely proportional to the cross-sectional area of the material. Hence, the magnetic reluctance of material decreases with an increasing cross-sectional area of the material.