Comprehensive Explanation (CCPS-based):

As temperature increases, the flammability range of a vapor widens , meaning it becomes easier for combustion to occur over a broader concentration range. This happens because higher temperatures provide additional energy to support combustion reactions.

The Lower Flammability Limit (LFL) decreases with increasing temperature. This means that less fuel is required to sustain combustion , making mixtures that were previously too lean now flammable.

At the same time, the Upper Flammability Limit (UFL) increases. This means that richer mixtures (higher fuel concentrations) can still support combustion because elevated temperatures improve reaction kinetics and reduce quenching effects.

Together, these changes expand the flammable range, increasing hazard potential. This is a critical concept emphasized by CCPS, particularly in evaluating process upsets, heated systems, and confined spaces , where elevated temperatures can significantly increase explosion risk.

Therefore, the correct answer is C: the upper limit increases and the lower limit decreases , indicating a widening of the flammable range.