Magnetic permeability is a fundamental property of a material that represents how easily a material can support the formation of a magnetic field within itself. Specifically, it is the measure of the ability of a material to allow magnetic flux to pass through it. This property is crucial in the design and functioning of transformers, inductors, magnetic circuits, and other electronic devices.

Let's analyze the options to determine the correct answer:

1. The ability of a material to resist the flow of electric current: This describes electrical resistivity, not magnetic permeability.
2. The ability of a material to oppose the flow of magnetic flux: This is more akin to the concept of magnetic reluctance, which is the opposition to magnetic flux.
3. The ability of a material to allow magnetic flux to pass through it: This describes magnetic permeability accurately. More precisely, a material with high magnetic permeability can easily channel magnetic lines of force.
4. The ability of a material to store energy in an electric field: This refers to electrical permittivity, related to the material's capacity to store electric energy.

This analysis confirms that the correct answer is: The ability of a material to allow magnetic flux to pass through it.

In practical terms, the magnetic permeability of a vacuum is denoted by \mu_0, also known as the permeability of free space, and its value is fixed by definition as approximately 4\pi \times 10^{-7} \, \text{N/A}^2. Materials with a permeability greater than that of a vacuum are called ferromagnetic materials, like iron.