The command ssh-add -t 3600 adds the private key to the SSH agent with an expiration time of 3600 seconds, which is equivalent to 1 hour. After this time, if not used, the key will be automatically removed from the agent, enhancing security by limiting the timeframe the key is available without user interaction. The options -x, -X, and -T do not serve the purpose described, either because they do not exist or -X unlocks the agent and -T is used for testing purposes in ssh itself, not ssh-add.