Which of the following is an example of an alloy?

An alloy is a combination of two or more elements, where at least one of the elements is a metal. This mixture results in materials that have improved properties compared to the individual elements. Brass, for example, is an alloy made primarily of copper and zinc. The combination of these metals gives brass desirable properties such as increased strength and improved resistance to corrosion, making it useful in various applications like musical instruments, plumbing fixtures, and decorative items.

In contrast, the other choices provided are either pure elements or non-metallic substances. Gold and iron are both pure metals, while oxygen is a non-metal gas, which do not meet the definition of an alloy as they are not combinations of multiple elements. Therefore, brass is correctly identified as the example of an alloy in this question.