(A) : If a conducting medium is placed between two charges, then electric force between them becomes zero.
(R) : Reduction in a force due to introduced material is inversely proportional to its dielectric constant.

When a conducting medium is placed between the two charges, then both charges behave like a single charge. Since electrostatic forces act only in between two charges the force will be zero in the present condition.

The assertion is true

$F_{medium}=\frac{F_{air}}{k}$

Therefore, when a material with dielectric constant k is introduced the force is reduced k times 

The reason is true

The correct option is A.