An officer’s direct observation of a crime provides ____________ cause for an arrest.

The term that best fits the scenario described in the question is "probable" cause. When an officer has direct observation of a crime, it means they have firsthand evidence or knowledge regarding the illegal activity taking place. This direct observation satisfies the legal standard of probable cause, which is the set of facts and circumstances that would lead a reasonable person to believe that a crime has been, is being, or will be committed.

Probable cause is essential for making an arrest, as it provides the necessary legal basis to detain an individual. This protects citizens from arbitrary arrests and ensures that law enforcement actions are guided by objective criteria. In contrast, the other terms do not adequately capture the legal threshold needed for an arrest based on an officer's direct observation. Exigent refers to circumstances that require immediate action, reasonable is a broader term that could pertain to a variety of situations beyond the legal context, and justifiable relates more to the justification of an action rather than the specific legal standard for arrest.