What is this constant (regarding electrostatics)

[NP04]

Homework Statement

Suppose that an electron could orbit a proton in a circular orbit of constant radius R. Assuming that the proton is stationary and only electrostatic forces act on the particles, which of the following represents the kinetic energy of the two particle system?

Relevant Equations

n/a

My only question is what the εo symbol represents.

 

[berkeman]

Problem Statement: Suppose that an electron could orbit a proton in a circular orbit of constant radius R. Assuming that the proton is stationary and only electrostatic forces act on the particles, which of the following represents the kinetic energy of the two particle system?
Relevant Equations: n/a

My only question is what the εo symbol represents.

The permittivity of free space/vacuum:

https://en.wikipedia.org/wiki/Permittivity

 

[NP04]

Thanks!