Final answer:
Diamond is a pure substance because it is composed entirely of carbon, making it an element. Granite is a heterogeneous mixture, and air is a homogeneous mixture of various gases.
Step-by-step explanation:
The student has asked which of the following is a pure substance: Diamond, Granite (quartz, feldspar and mica), or air. To identify pure substances, we refer to materials that have a consistent composition and characteristic properties.
Diamond qualifies as a pure substance as it comprises solely of carbon and is an example of an element. On the other hand, granite is a heterogeneous mixture of quartz, feldspar, and mica; each can be identified by their distinct properties within the mixture. Similarly, air is not a pure substance but a homogeneous mixture comprised of gases like nitrogen, oxygen, water vapor, carbon dioxide, among others, in variable proportions depending on the location.