The alarm stage of stress is when the body prepares itself for

relief of muscle tension
repair of the damage done by the previous stage
fight-or-flight
a decrease in blood pressure

Fight-or-flight

Explanation:

Fight-or-flight is an immediate response of the brain and body when detecting threats or other dangerous situations in the environment, so it prepares the person to confront or run away from the potential source of danger.

When the ¨alarm stage¨ starts, it automatically provokes an immediate reaction in the individual against the external threat, this way the body gets fully prepared in order to avoid any effect or contact with it.