Which of the following is NOT an example of an alloy?

An alloy is defined as a combination of two or more metals or a metal and another element, which results in improved properties compared to the individual components. Steel and brass are prime examples of alloys, as they are made from mixtures of metals: steel is primarily iron with carbon, while brass consists of copper and zinc.

Aluminum, in its pure form, is a metallic element rather than an alloy, although it can also be mixed with other metals to create various alloys. However, while aluminum can exist as an alloy, the question specifically asks for what is "NOT" an alloy.

Silver is a precious metal and, in its pure state, is not an alloy; it is used in its elemental form, devoid of any other metallic fusion. Therefore, identifying silver as the option that is not an alloy is appropriate, as it does not fit into the category of intentional mixtures that enhance certain characteristics found in alloys.