The correct answer is C. Collective Security.
Step-by-step explanation
Collective security is a concept that refers to a type of political alliance between countries that agree to maintain security among all. Therefore, they undertake to act collectively when there is a threat to any of them. This idea is related to the phrase in the fragment "The parties agree that an attack against one or more of them shall be considered an attack against all." So the correct answer is C. Collective Security.