In free space, the parameter that will have a value of unity is the relative permittivity (also known as the dielectric constant).

Explanation:
- Relative permittivity (εᵣ) is a dimensionless quantity that represents the ratio of the permittivity of a material to the permittivity of free space (ε₀).
- Permittivity (ε) is a property of a material that describes its ability to store electrical energy in an electric field. It is measured in farads per meter (F/m).
- Absolute permittivity (ε) is the permittivity of a material, which is a physical constant and has a specific value depending on the material.
- Permeability (μ) is a property of a material that describes its ability to support the formation of a magnetic field within itself when an electric current is flowing. It is measured in henries per meter (H/m).

In free space, the relative permittivity is equal to 1, indicating that the material has the same ability to store electrical energy as free space. This means that the electric field within free space is not affected or distorted by any material present, and it behaves as if there is no material present.

The value of the relative permittivity determines the speed at which electromagnetic waves propagate through a material. In free space, electromagnetic waves (such as light) travel at the speed of light, which is approximately 3 x 10^8 meters per second. This is because the relative permittivity of free space is unity, resulting in no delay or distortion of the electromagnetic waves.

In practical terms, the value of relative permittivity is used to characterize the behavior of materials in electrical and electronic systems. Different materials have different relative permittivity values, and this property is important for designing and analyzing various components such as capacitors, transmission lines, and antennas.

Therefore, in free space, the parameter that will be unity is the relative permittivity (option C).