Which of the following materials is considered an alloy?

Stainless steel is considered an alloy because it is comprised of a mixture of metals, primarily iron, with significant additions of chromium and nickel, among other elements. This combination enhances its properties, making it more resistant to corrosion and staining compared to pure iron. Alloys are characterized by their improved physical properties, including strength and durability, which often surpass those of the individual metals that comprise them.

The other materials listed are not classified as alloys. Pure iron is simply iron without additional elements, while gold in its typical form is also a pure metal. Clay, on the other hand, is a natural material made up of minerals, rather than a metallic mixture, and does not fit the definition of an alloy. Thus, stainless steel stands out as the correct answer due to its composite nature and resultant enhanced characteristics.