Which mixture is specifically designed to serve a useful function?

The correct choice is a formulation because it is a specific mixture designed to have a particular function. Formulations are carefully designed combinations of different substances that all contribute to the intended use. These can be found in various products like medicines, paints, cleaning products, and fuels, where each component plays a vital role in the effectiveness and safety of the final product.

For example, in pharmaceuticals, a formulation may include the active ingredient along with excipients that aid in the drug's delivery, stability, and absorption in the body. This intentional design contrasts with the other options, which do not necessarily imply a purposeful mixture with a specific function. A chemical reaction involves the transformation of substances rather than a crafted mixture, while elements are pure substances that cannot be separated into simpler substances, and compounds consist of two or more elements that are chemically bonded together without any deliberate design for a particular purpose.