Which condition must exist for an arrest to be justified?

For an arrest to be justified, the presence of a crime is a fundamental condition. Law enforcement officers must have probable cause to believe that a crime has been committed, and this belief must be based on factual evidence or observable circumstances. The existence of a crime underlies the authority of an officer to take someone into custody, ensuring that the action is not arbitrary or unfounded.

The other aspects mentioned, like the lack of witnesses, the presence of a firearm, or an officer's discretion, do not inherently justify an arrest. The mere absence of witnesses does not negate the potential for an arrest if there is probable cause based on other evidence observed by the officer. Similarly, the presence of a firearm might be relevant in specific situations but does not automatically correlate with the justification for an arrest. Lastly, while an officer's discretion plays a role in law enforcement actions, it must still align with the existence of a crime for an arrest to be legally sound.