Final answer:
The distress signal used to indicate a grave and imminent life-threatening danger is SOS.
Step-by-step explanation:
A distress signal used to indicate a grave and imminent life-threatening danger is SOS.
In physics, SOS is an internationally recognized Morse code signal consisting of three short signals, three long signals, and three short signals. It is used as a distress signal to request immediate assistance when there is an emergency or life-threatening situation.
For example, ships at sea or airplanes in distress can use the SOS signal to communicate their urgent need for help.