In material selection for a mechanical process, which of the following is a composite material?

Concrete is classified as a composite material because it is made up of multiple components that come together to create a material with specific properties distinct from those of its individual parts. Typically, concrete consists of a mixture of cement, water, aggregates (such as sand and gravel), and sometimes additional additives. The cement acts as a binder that holds the aggregates together, providing strength and durability to the structure.

In comparison, steel, aluminum, and iron are all classified as pure metals. They are not composites because they consist of single metallic elements rather than a combination of materials. While alloys, which are mixtures of different metals, can be considered composite in some contexts, in general discussions around composite materials, attention often focuses on mixtures that combine dissimilar materials. Therefore, concrete is the correct answer as it exemplifies the characteristics and benefits of a composite material effectively.