Which statement describes heterogeneous mixtures?
a. they are made up of two or more pure substances that are not chemically bonded together.
b. they appear uniform, like a pure substance.
c. they have fixed ratios between their components.
d. they are made up of two or more pure substances that can be separated only by chemical processes.

The righ answer is the option A. They are made up of two or more pure substances that are not chemically bonded. A classicall expample ot heterogeneous mixtures are sugar and salt. Sugar is a pure substance, salt is also a pure substance, when you mixe them you form mixture where salt is still salt and sugar is still sugar, that is what "they are not chemically bonded" means. So you can separate them by physical media.