Answer: C. Emergency lockdown.
Explanation: Emergency lockdown is a form of response action that is taken when there is an active threat such as a shooter or a hostage taker situation. It involves moving everybody present on the premises to a relatively safe location, locking all doors, ceasing all activities that can call attention to the hiding place, switching off all lights and computer monitors and remaining quiet.
If you are able to contact law enforcement, it should be done quietly so as not to draw attention to your hiding place.
In contrast, reverse evacuation involves moving those present outside inside because of the presence of an aggressor outside. You can then call law enforcement from inside the building.