If a police officer hears a scream and enters a house without a warrant, under what circumstance can they legally make an arrest?

The correct choice is grounded in the concept of "exigent circumstances," which allows law enforcement officers to act without a warrant when there is an immediate need to protect life or prevent serious injury. If an officer hears a scream, it can indicate that someone may be in danger or experiencing a life-threatening situation. In such scenarios, the law recognizes that obtaining a warrant might take too long, posing a risk to life.

When the officer believes someone's life is in danger, they are justified in entering the premises to ensure safety and, if necessary, make an arrest if they observe a crime or imminent threat. This principle is essential for balancing individual rights with the need for public safety.

Other types of circumstances mentioned, such as having a warrant or prior knowledge of illegal activity, do not justify immediate entry without a warrant. Likewise, witnessing a crime happening is also a valid basis for immediate action, but it is not the only context where entry can be justified without a warrant; the perception of imminent danger, as outlined, stands on its own.