Final answer:
Wood is the substance among the given options that is a mixture, specifically a heterogeneous mixture, due to its varied composition of organic compounds and potential inclusion of minerals and water.
Step-by-step explanation:
Among the options given (water, chromium, wood, sulfur), wood is the correct answer for the substance that is a mixture. Wood consists of cellulose, hemicellulose, lignin, and other organic compounds and can contain different minerals and water. This composition makes wood a heterogeneous mixture because its components are not uniformly distributed throughout.
To contrast, water (H2O) is a compound as it is made up of two hydrogen atoms bonded to one oxygen atom in a fixed ratio. Chromium is an element, and pure sulfur is also an element, both consisting of a single type of atom.
Other examples of heterogeneous mixtures include soil, which is made up of various materials in different proportions, and not all components are distributed evenly throughout.