The lowest temperature at which a flammable liquid gives off enough vapor to form an ignitable mixture in air describes or defines its

 Flash Point

Explanation:  Volatile substance is the substance which can easily catch fires.

Thus, there must be a point to define that temperature when the volatile substance catches fire given enough source of flame. This point is known as Flash Point.

Thus, Flash point is defined as the lowest temperature at which the flammable or volatile liquid catches fire and convert itself into the ignitable mixture when enough source of ignition is given.

the answer is a. co2, this would be colder than the : )

explanation: