Final answer:
Methane (CH₄) is the only gas among those listed that will ignite at the lowest concentration in air, as it is a flammable hydrocarbon.
Step-by-step explanation:
The question is asking which of the listed gases will ignite at the lowest concentration in air. Among the options provided, methane (CH₄) is the only flammable gas and will ignite at the lowest concentration when mixed with air. Oxygen (O₂), nitrogen (N₂), and carbon dioxide (CO₂) are not flammable gases. Nitrogen, which makes up a majority of the atmosphere, is inert and dilutes oxygen to prevent rapid combustion. Oxygen is necessary for combustion but does not ignite on its own. Carbon dioxide is commonly used as a fire extinguisher and does not ignite. Methane, however, is a highly flammable hydrocarbon and is capable of igniting at a low concentration when mixed with the oxygen in the air.