Under what circumstances can police arrest a person without a warrant?

The correct answer is based on the understanding of circumstances in which police can make arrests without a warrant, particularly when it aligns with the concept of public interest and safety. Police may conduct a warrantless arrest when they reasonably believe it is necessary to prevent further harm, to preserve evidence, or to ensure that the suspect appears for court proceedings in a timely manner. This aligns with the duty of law enforcement to uphold public safety and order, suggesting that considerations of public interest play a crucial role in the decision to arrest without a warrant.

In contrast, the other options do not accurately reflect legal standards for warrantless arrests. Uncertainty about a suspect does not justify an arrest; rather, clear and articulable evidence of wrongdoing is typically required. A suspect's cooperation does not negate the need for probable cause; police require a legitimate reason to proceed with an arrest. Lastly, witnessing a crime is a common ground for arrest without a warrant, but the expectation of police action extends beyond simply witnessing a crime to include situations where there is impending risk to public safety or potential loss of evidence that necessitates immediate action. Thus, while witnessing a crime may provide a basis for an arrest, it is not the only circumstance under which police can arrest a person without a warrant.