With the first two words "treaty" and "war" paired, then the assumption can be the opposite of each other. For wars and treaty go hand in hand, the treaty being the opposite of the war, which is like peace and trouble put together.
So also, sleep will be the opposite of an "alarm clock" for alarms only disturb sleep or prevents one from sleeping. The other options given all allude to the act of getting some peace or rest. Thus, the right answer will be "alarm clock: sleep".
