Answer:
a.Hiding a key outside to ensure family members can get in if they lose their keys.
Explanation:
According to the Antiterrorism Scenario Training
Hiding a key outside to ensure family members can get in if they lose their keys is not a physical security measure for the home because of the reason that this risks and increases the chances of getting an intruder to get access in the building.
The intruder may find these keys and can get an easy and safe access in the home.