A pure substance can only be:

Pure substances are those that have a fixed composition and exhibit the same properties even in different places and at different times.

Pure substances include elements and compounds.

•  Element: They are made up of a single type of atom. All elements are represented by a chemical symbol and are arranged in order of increasing atomic number in the periodic table.

Example-$\mathrm{He}, \mathrm{Na}, \mathrm{B}, {\mathrm{F}}_2$

• Compound: Compounds are homogeneous substances formed by combining 2 or more elements in a fixed mass ratio. The basic elements of a compound cannot be extracted by physical methods. The properties of compounds are completely different from their constituent particles.

Example - ${\mathrm{H}}_2\mathrm{O}, {\mathrm{CO}}_2, {\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6, {\mathrm{SiO}}_2.$

So the correct option is C).