Which of the following is considered a flammability characteristic?

Vapour pressure is indeed a crucial flammability characteristic because it indicates the tendency of a substance to evaporate and produce vapours at a given temperature. High vapour pressure signifies that a compound can easily form vapours, which can mix with air and potentially ignite if there is an appropriate ignition source. This makes it an important factor in assessing the flammability of liquids and gases.

The other characteristics listed do not relate directly to flammability in the same way. Salinity refers to the concentration of salts in a solution and does not impact the flammability of a substance. The melting point is a physical property that determines the temperature at which a solid turns into a liquid, while reactivity refers to how a substance interacts chemically with others; neither of these directly speaks to the potential for ignition or burning.