An example of a heterogeneous mixture is

The correct choice, sand and water, is an example of a heterogeneous mixture because it consists of different components that are not uniformly distributed. In this case, when sand is mixed with water, the sand particles do not dissolve but instead remain as separate, visible grains; the water acts as a liquid medium surrounding these grains. This distinct separation allows for the identification of individual components, which is characteristic of heterogeneous mixtures.

In contrast, soda water is a homogeneous mixture as the carbon dioxide is dissolved uniformly throughout the water, creating a consistent solution. Acid typically refers to a chemical compound and can be either homogeneous or heterogeneous depending on the specific context, but commonly, solutions of acids are homogeneous. Flue gas, while it contains multiple gases, is generally considered a homogeneous mixture in terms of composition as the gases mix thoroughly at a molecular level.