The correct answer is: "collective security".
The collective security policy refers to the establishment of a treaty, agreement or arrangement at a local, regional or global level, given that its principal aim is to maintain security for all the participants and to face together the possible common threats. One sucessful example is the United Nations, although its precedessor, the League of Nations was unsucessful as it could not prevent the start of WWII.
The appeasement policy, in the international sphere, consits on making political or material concessions to an aggressive country or organization in order to avoid starting a conflict (war). For example, European Allied powers (UK and France) made territorial concessions to Hitler as, after WWI, they wanted to avoid a new war by all means. Hitler managed to obtain Czechoslovakia with no resistance. But he was not willing to stop there and in the end the Allied powers had to declare war, but before they had allowed Hitler to became stronger with no opposition.
Therefore, it seems more effective to use the collective security system in order to maintain peace, as long as its grounds are strong enough to restrain defecting behaviours on its members. The membership feeling usually helps that all participants regard themselves as part of the collective and not willing to break its bonds or protection.
