监守者

例句：

1. 1. The prisoner was under the constant supervision of a guard.
   这名囚犯一直由一名警卫严密监视。
2. 2. The captain is the guardian of the ship's safety.
   船长是船只安全的守护者。
3. 3. The watchman kept a close eye on the suspicious activity in the park.
   看门人密切监视公园里可疑的活动。