The correct answer is A. Reluctance.
Reluctance is the opposition to the flow of magnetic flux in a magnetic circuit. It is analogous to resistance in an electrical circuit. The lower the reluctance, the easier it is for magnetic flux to flow.
Permeability is a measure of how easily a material can be magnetized. It is a property of the material itself, and it does not depend on the shape or size of the material.
Permeance is the product of permeability and cross-sectional area. It is a measure of how much magnetic flux can be produced in a material.
Retentivity is the ability of a material to retain its magnetism after the magnetizing force has been removed.
There is no valid comparison between reluctance and the other options.