As explained by Roberts, which of the following would be an example of "collective security"?

a. Each state in the system pledges not to attack any other state in the system.
b. All states in the system merge their military forces to create a system-wide army.
c. All states in the system agree that they will join with an aggressor in an attack against a victim.
d. All states in the system agree that if any one state is attacked, the other will join forces with the victim of the attack.
e. A group of states join together against an external threat. They pledge to come to each other's aid in the event of an attack by the threatening state.

Answer: d