A

Step 1: Determine the force acting on the plates due to electrostatic attraction.
The electrostatic force (F) between two charges can be calculated using Coulomb's Law:
$$ F = \frac{1}{4\pi\epsilon_0} \cdot \frac{q^2}{r^2} $$
where:
- $F$ is the force between the charges
- $q$ is the charge on the plates
- $\epsilon_0$ is the permittivity of free space
- $r$ is the separation between the plates

Step 2: Relate the electrostatic force to the spring force.
The spring force due to extension (x) is given by Hooke's Law:
$$ F = kx $$
where:
- $k$ is the force constant of the spring

Step 3: At equilibrium, the magnitudes of the electrostatic force and the spring force must be equal.
Therefore:
$$ kx = \frac{1}{4\pi\epsilon_0} \cdot \frac{q^2}{r^2} $$
Step 4: Solving for the extension of the spring (x), we get:
$$ x = \frac{q^2}{4\pi\epsilon_0 k r^2} $$
The equilibrium extension in the spring can be expressed with respect to the given parameters. Therefore, the extension in the spring in equilibrium is option A.