Which of the following ions have -2 valency?

Valency is the combining capacity of the elements to form compounds.

The sulphate ion is derived from sulphuric acid (H₂SO₄). It is combined with two hydrogens in this compound. Hence, we can say that the combining capacity of sulphate is 2.

So, the valency of sulphate ion =-2

(Valency of hydroxide ion =-1, Valency of Nitrate ion =-1, valency of ca ion= +2)

Hence, sulphate ion is the correct answer.