Which of the following is an element?

The correct option is B

Oxygen

Explanation for correct option:

B. Oxygen

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 $\left(\mathrm{Na}\right)$, and Oxygen $\left(\mathrm{O}\right)$.
• Oxygen is a chemical element with an atomic number $8$.

Explanation for incorrect options:

A: Water$\left({\mathrm{H}}_2\mathrm{O}\right)$ is a chemical compound(“A compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion.”) as it is made up of two elements i.e. Hydrogen and oxygen.

C: Salt water is a mixture(“A mixture contains more than one substance (element and/or compound) mixed in any proportion.”)

Hence, the correct option is B i.e. Oxygen.