_________ should not be placed near an ignition source because it is explosive.

Propane should not be placed near an ignition source because it is a highly flammable substance that can easily ignite and lead to explosive situations. Propane is a common fuel used in various applications, including heating and cooking, but when exposed to heat or an open flame, it can ignite rapidly. This characteristic makes it critical to store propane safely and away from any potential ignition sources to prevent the risk of fire or explosion.

In contrast, oxygen, while it supports combustion, is not flammable by itself; it requires a fuel to burn. Argon and neon are inert gases, meaning they do not react readily with other substances, including ignition sources. Their chemical properties reduce the likelihood of causing explosions or fires, making them safe to be near potential ignition sources. Thus, the distinction of propane as a flammable substance is the defining factor supporting its classification as needing careful storage away from ignition hazards.