Absolute permeability is the constant that relates the magnetic flux density in a material to the magnetic field strength. It is a fundamental property of a material and is denoted by the symbol μ. It is defined as the ratio of the magnetic flux density (B) to the magnetic field strength (H) in a material.

The formula for absolute permeability is given by:
μ = B/H

Explanation of the options:
a) Absolute Permeability: This is the correct answer. It is the constant that relates B to H in a material.
b) Coefficient of Mutual Induction: This refers to the ability of two coils to induce a voltage in each other when the current in one coil changes. It is denoted by the symbol M.
c) Coefficient of Self Induction: This refers to the ability of a coil to induce a voltage in itself when the current flowing through it changes. It is denoted by the symbol L.
d) Relative Permeability: This is a dimensionless quantity that compares the permeability of a material to the permeability of free space (μ0). It is denoted by the symbol μr.

Relative Permeability can be calculated using the formula:
μr = μ/μ0

Conclusion:
In conclusion, the constant that is called Absolute Permeability is denoted by the symbol μ and relates the magnetic flux density to the magnetic field strength in a material. It is a fundamental property of a material and is used to describe its magnetic properties. The other options mentioned, such as the Coefficient of Mutual Induction, Coefficient of Self Induction, and Relative Permeability, are related concepts but not the correct answer to this question.