A non-magnetic ore containing the impurity of ${\mathrm{FeCr}}_2{\mathrm{O}}_4$ is concentrated by:

A non-magnetic ore containing the impurity of  ${\mathrm{FeCr}}_2{\mathrm{O}}_4$  is concentrated by magnetic separation. ${\mathrm{FeCr}}_2{\mathrm{O}}_4$ is a ferromagnetic material, hence magnetic separation is used to concentrate an ore of  ${\mathrm{FeCr}}_2{\mathrm{O}}_4$ that contains tin. Magnetic separation is based on variations in the magnetic characteristics of the ore constituents. 

Hence, option A is correct