Which of the following is NOT a type of alloy?

The option identified as the correct answer, which is plastic, accurately reflects that it is not an alloy. Alloys are specifically defined as a combination of two or more metals, or a metal and another element, that has metallic properties. Steel, brass, and stainless steel are all examples of alloys.

Steel is primarily a mixture of iron and carbon, while stainless steel includes iron, carbon, and chromium, which enhances its resistance to corrosion. Brass is another alloy, primarily made of copper and zinc, and is valued for its machinability and its acoustic properties.

In contrast, plastic is a synthetic material made from polymers that can be derived from natural sources or synthesized; it does not consist of metals and does not exhibit metallic properties. Therefore, classifying plastic as an alloy is incorrect, which supports the choice made for this question.