Which of the following is an example of a heterogeneous mixture?

A heterogeneous mixture is characterized by a composition that is not uniform throughout, meaning that the different components can be observed and often separated. In the case of an oil and water emulsion, the distinct layers of oil and water can be seen, and the two substances do not mix on a molecular level. This results in the formation of two separate phases, clearly indicating that it is a heterogeneous mixture.

Oil and water do not form a single solution; instead, they exist as separate entities within the mixture. This visual and physical separation exemplifies the nature of a heterogeneous mixture, as opposed to a homogeneous mixture where the components are uniformly distributed and indistinguishable.

In contrast, other options like acid, flue gas, and metal alloys are either homogeneous or more uniform mixtures. Acid solutions, for example, typically consist of dissolved substances and appear uniform throughout. Flue gas contains numerous gases mixed thoroughly, making it homogeneous. Metal alloys generally result in a uniform distribution of metals, leading to a consistent composition. These characteristics clarify why they do not fit the definition of a heterogeneous mixture like an oil and water emulsion does.