Which of the following is not an element?

The correct option is C

Silica

Explanation for correct option:

C. Silica

Elements:

• “An element is a basic form of matter that cannot be broken down into simpler substances by chemical reactions.”
• Elements can be metals/non-metals/metalloids.
• Examples of elements are Iron $\left(\mathrm{Fe}\right)$, Sodium $\left(\mathrm{Na}\right)$, or Gold $\left(\mathrm{Au}\right)$.

Silica:

• Silica is not an element rather it is a compound that is composed of silicon and oxygen atoms.
• The chemical formula of silica is ${\mathrm{SiO}}_2$.
• Silica is chemically known as Silicon dioxide.

Explanation for incorrect options:

A. Graphite is a chemical element that is made up of carbon.

B. Germanium is a chemical element with an atomic symbol $\mathrm{Ge}$.

D. Silicon is a chemical element having an atomic symbol $\mathrm{Si}$.

Hence, the correct option is C i.e. Silica.