Find the elements which show variable valency:
A.Copper
B.Sodium
C.Calcium
D.Magnesium

Hint: We know that the electrons present on the outermost shell of an atom are called valence electrons. During the chemical reaction, these electrons can be gained or donated.

Complete step by step answer:
First, we understand the valency.
Valency is the number of electrons accepted or donated by an atom to attain the stable electronic configuration of the nearest inert gas.
Now, let us discuss the variable valency.
Variable valency is generally found in transition elements of the periodic table and it happens as the second last shell of transition metal and it is not filled with electrons.
Let us understand why elements exhibit variable valency.
Some of the transition elements show variable valency because of the different electronic configuration. Sometimes an atom of an element loses its electron from the penultimate shell and thus has more than $1$ valency.
For example, Iron forms ferric and ferrous oxide. Iron has $ + 2$ oxidation state at ferrous oxide whereas iron shows $ + 3$ oxidation state at ferric oxide. This is called variable valency.
Now going to the options (B), (C), and (D). Sodium, calcium, and magnesium belong to the s-group so they do not show variable valency. Hence options B), (C), and (D are incorrect.
Copper is a d-block element so it shows variable valency. Copper forms a compound containing $C{u^ + }$ and $C{u^{2 + }}$ ions.

Hence option (A) is correct.

Note:
We must know that the capability of the transition metal to show variable valency is usually attributed to the availability of more electrons in the $\left( {n - 1} \right)d$ orbital which is closer to the outermost $ns$ orbital in energy levels.