Which of the following elements does not show variable valency?

The correct option is C

Hydrogen

Variable valency

The combining capacity of an element is called valency. Some elements have more than one valency because they combine with other elements to form different compounds. These elements have variable valency.

Explanation for the correct option

(c) Hydrogen

It has only one valence shell. Therefore, the valency of hydrogen is 1. It cannot show variable valency.

Explanation for incorrect option

(a) Mercury

Mercury reacts with chlorine to form two different compounds Mercury(I) chloride$\left({\mathrm{Hg}}_2{\mathrm{Cl}}_2\right)$ and Mercury (II) chloride$\left({\mathrm{HgCl}}_2\right)$

(b) Copper

Copper reacts with oxygen to form copper(I) oxide$\left({\mathrm{Cu}}_2\mathrm{O}\right)$ and Copper(II) oxide$\left(\mathrm{CuO}\right)$

(d) Iron

Iron can combine with chlorine to form two different compounds Ferrous chloride$\left({\mathrm{FeCl}}_2\right)$ and Ferric chloride$\left({\mathrm{FeCl}}_3\right)$. Therefore, iron show variable valency.

Therefore, Hydrogen is the element that does not show variable valency. Hence, option(c) is correct.