The equilibrium Cr_2O_7^2- + 2e^- ⇌ 2CrO_4^2-

The answer is (1), Exists in basic medium.

In basic medium, the chromate ion \(2CrO_4^{2-}\) is reduced to the dichromate ion \(Cr_2O_7^{2-}\). This reduction is accompanied by the gain of two electrons, which are lost by the chromate ion.

The equilibrium is favored in basic medium because the dichromate ion is more stable in basic solution. In acidic medium, the dichromate ion is converted to the chromate ion, which is less stable.