Which of the following elements shows variable valency?
A.) Iron
B.) Silver
C.) Zinc
D.) Potassium

Hint: Valence electrons are those electrons found in the outermost shell of an atom. In other words, these are the electrons that can be gained or lost during a chemical reaction.

Complete step by step answer:
Before knowing about variable valency let us first understand about valency.
Valency is the number of electrons gained or lost by an atom in order to become stable by attaining the electronic configuration of the nearest noble gas.

Now, let us know about variable valency.
Variable valency is a phenomenon usually found in transition elements of the modern periodic table. It occurs as the second last shell of these elements and is not completely filled with electrons.

Let us understand why elements show variable valency.

Some elements show variable valency because of the different electronic configurations. An atom of an element can sometimes lose more electrons than are present in its valence shell i.e. loss from the penultimate shell and hence exhibit more than 1 or variable valency.
For example, iron combines with oxygen to form ferrous oxide as well as ferric oxide. In the formation of ferrous oxide, iron exhibits a valency of +2, whereas in ferric oxide, it has a valency of +3. This is termed variable valency.

Hence, Iron valency shows variable valency. Therefore, option A is the required answer.

Note: The ability of the transition metals to exhibit variable valency is generally attributed to the availability of more electrons in the (n-1)d orbital which are closer to the outermost ns orbital in energy levels.