What is the definition of an explosive atmosphere?

An explosive atmosphere is defined as a mixture of air and flammable substances in which, under certain conditions, combustion may occur, leading to an explosion. This includes anything that can ignite easily, such as gases, vapors, or dusts, when mixed with air in the right proportions.

In the context of the other options, the first choice refers to non-flammable substances, which do not contribute to ignition or explosion potential. The third choice involves inert gases, which do not support combustion and therefore cannot create an explosive atmosphere with air. The fourth option describes a mixture involving solid or liquid substances, but without specifying their flammability in relation to air, it does not adequately convey the concept of an explosive atmosphere. Thus, only the option that highlights the presence of flammable substances is aligned with the established definition of an explosive atmosphere.