Which of the following is an example of an element?

Oxygen is classified as an element because it is a pure substance that cannot be broken down into simpler substances by chemical means. It is represented by the chemical symbol 'O' and is one of the fundamental building blocks of matter. Elements are defined as substances that consist of only one type of atom and cannot be split into smaller components while retaining their chemical properties.

In contrast, air is a mixture of various gases, including oxygen, nitrogen, and carbon dioxide, and therefore does not qualify as a pure element. Water and salt are compounds made from two or more elements combined in fixed ratios, meaning they can be broken down into their respective elements – hydrogen and oxygen in the case of water, and sodium and chlorine in the case of salt. Thus, these do not fit the definition of an element, making oxygen the correct answer.