Which of the following is pure substance?

Pure substances are those that have fixed composition and show the same properties even at different places and at different times. Pure substances are classified as Elements and compounds.

Mixtures are formed when two elements or compounds are combined physically and not chemically.

As air consists of nitrogen, oxygen, carbon dioxide, methane, argon, and water vapors, it is considered a mixture.

Rock is found to be a mixture of different minerals and organic matter.

Diamond is an allotropic form of carbon. Thus, it is a pure compound of only carbon.

Steel is an alloy of iron and carbon. As it consists of two types of elements, it is a mixture.

Hence, only diamond is a pure substance in the given options.

Thus, the correct option is C.