The property of a medium which determines the electric force between two charges situated in the medium is

Permittivity

The correct answer is Option (2) → Permittivity

The electric force between two charges in a medium is given by Coulomb's law:

$F = \frac{1}{4 \pi \varepsilon} \frac{q_1 q_2}{r^2}$

Here, $\varepsilon$ is the permittivity of the medium. A higher permittivity reduces the force between charges. Therefore, the property of a medium that affects the electric force is its permittivity.