The valency of an element refers to its ability to form chemical bonds with other elements. It is determined by the number of electrons an atom gains, loses, or shares during a chemical reaction.

In option 1, the elements Na, Mg, and Ca have different valencies. Na has a valency of +1, Mg has a valency of +2, and Ca has a valency of +2. Therefore, option 1 is not correct.

In option 2, the elements Na, Mg, and Al also have different valencies. Na has a valency of +1, Mg has a valency of +2, and Al has a valency of +3. Therefore, option 2 is not correct.

In option 3, the elements Mg, Ca, and K have different valencies. Mg has a valency of +2, Ca has a valency of +2, and K has a valency of +1. Therefore, option 3 is not correct.

In option 4, the elements Mg, Ca, and Ba have the same valencies. Mg has a valency of +2, Ca has a valency of +2, and Ba has a valency of +2. Therefore, option