The correct answer is A. 1.
The relative permittivity of free space is a dimensionless constant that is equal to 1. It is denoted by $\epsilon_0$. The relative permittivity of a material is a measure of how much the electric field is distorted by the presence of the material. The relative permittivity of free space is the reference value against which the relative permittivity of other materials is measured.
The relative permittivity of free space is important in electromagnetic theory. It is used to calculate the speed of light in a vacuum, the electric field strength in a material, and the capacitance of a capacitor.
The relative permittivity of free space is also known as the permittivity of vacuum or the permittivity of free space.
Option B is incorrect because the relative permittivity of free space is not equal to 10.
Option C is incorrect because the relative permittivity of free space is not equal to 100.
Option D is incorrect because the relative permittivity of free space is not equal to 1000.