A flash fire during hot work is a sudden, fast-spreading emergency near flammable material, so the governing safety principle is life first: warn everyone and get people to safety before anything else.

The correct immediate response is therefore to activate the nearest fire alarm and evacuate the area following the emergency procedures. Sounding the alarm alerts all workers at once and summons trained fire responders, while orderly evacuation removes people from danger.

Trying to hide the incident to avoid interrupting work leaves the fire to grow unchecked and denies everyone warning, turning a small fire into a disaster.

Completing the current hot work first ignores the emergency and wastes the critical early seconds when a fire is most controllable and escape is easiest.

Attempting to fight the flash fire alone before informing anyone leaves others unwarned and puts the lone worker at grave risk if the fire spreads faster than expected; raising the alarm must come first.

The action that protects lives and brings help immediately is to alarm and evacuate.

Hence, the answer is Activating the nearest fire alarm and evacuating the area according to emergency procedures.