Final answer:
A mixture is a physical blend of two or more pure substances, where each substance retains its own identity and properties and can be separated by physical methods. Mixtures are not in a fixed ratio and they can be either homogeneous or heterogeneous.
Step-by-step explanation:
The correct definition of a mixture is A) a mixture is two or more pure substances combined. Mixtures are material systems consisting of two or more different substances which are mixed but are not combined chemically. A mixture refers to the physical combination of two or more substances in which the identities are retained and are mixed in the form of solutions, suspensions, or colloids. For example, sand and iron filings can be mixed together but can easily be separated by a magnet.
Homogeneous mixtures have the same composition throughout, such as salt dissolved in water, whereas heterogeneous mixtures contain different substances that are not uniform in composition, like a salad dressing of oil and vinegar.