Understand the definition of a pure substance: A pure substance has a fixed composition and distinct chemical properties. It can be either an element or a compound, but not a mixture.
Analyze each option to determine if it is a pure substance or a mixture:
Sea water is a mixture because it contains water, salts, and other dissolved substances in varying amounts.
Brass is a mixture (an alloy) composed mainly of copper and zinc, so its composition can vary.
Air is a mixture of gases such as nitrogen, oxygen, carbon dioxide, and others, with variable composition depending on location and conditions.
NaCl (sodium chloride) is a pure substance because it is a chemical compound with a fixed ratio of sodium and chlorine atoms, forming a crystalline solid with consistent properties.