Which term indicates a mixture that includes multiple plant species?

The term that best indicates a mixture that includes multiple plant species is "mixture." This term broadly describes a combination of different components, which, in this context, refers specifically to various plant species combined together.

When considering the other options, "tea" typically refers to a beverage made by brewing leaves—often from a single plant species or a carefully selected blend but does not inherently suggest multiple species in a broader sense. "Extract" refers to a concentrated substance derived from a plant and often represents a single species, or a concentrated form of its active ingredients, rather than a blend of multiple species. "Granules" are a physical form and do not correlate directly to the concept of including multiple plant species; they could refer to any granulated substance, including those from a single plant source, without specifying the diversity of species involved.

Thus, "mixture" effectively encompasses the idea of uniting multiple plant species, making it the most fitting choice for this question.