Which element is essential for the burning process in flammable vapors?

Oxygen is essential for the burning process in flammable vapors because it is a key component of combustion reactions. During combustion, a fuel (like flammable vapors) reacts with oxygen, resulting in the release of energy in the form of heat and light. This process requires a sufficient concentration of oxygen to sustain the reaction; without it, combustion cannot occur.

In normal atmospheric conditions, oxygen comprises about 21% of the air, providing the necessary environment for flames to ignite and continue burning. If the oxygen level is too low, the fire will extinguish. Thus, oxygen is indispensable in the burning process, making it the correct choice in this context.