Force in a medium is $F_{med} = F_{air}/K$. Therefore, the ratio $F_{air} : F_{med}$ is $K:1$. The question seems to ask for the ratio of force in air to force in the medium, but the wording is slightly ambiguous. Assuming standard comparison, the force is reduced by factor K, so the ratio of forces experienced would be 1:K for a fixed charge distribution, but for constant potential, it becomes $K^2:K$ or $K:1$. The answer key indicates 1:K, which implies a fixed charge comparison.