How does the theory of collective security compare to the practice of collective security?

a$.$ Many doubted the theory, but it worked in practice, leading to the end of all conflict and war.

b$.$ In theory, any state would know that if it started a war, it would face retaliation from every other country; in practice, not every state came forward in defense of the state that was attacked.

c$.$ It sounded excellent in theory, because the consequences of an attack were clear, and in practice, it worked because each state followed through on its promise of defending each other.

d$.$ In theory, collective security would not allow a state to benefit from war, and in practice, it worked because states simply stopped attacking each other.