No certificate holder may allow any person to board any of its aircraft if that person ___________ to be intoxicated.

The correct choice focuses on the term "appears." This language is significant in the context of aviation regulations as it emphasizes the observable condition of a person regarding intoxication rather than relying solely on their own assertions or claims.

In aviation safety protocols, ensuring that all passengers are fit to fly is paramount. The regulation recognizes that visual cues and observable behaviors are important indicators of a person's state. If a person appears to be intoxicated, it is the responsibility of the certificate holder to prevent them from boarding the aircraft to ensure the safety of all passengers and crew.

The other terms like "claims," "seems," and "looks" do not hold the same weight as "appears" in regulatory language. "Claims" would imply that the individual is verbally stating they are intoxicated, which does not align with the intent of preventing unsafe conditions. "Seems" and "looks" may suggest a degree of uncertainty or subjectivity, which does not align with the clear and definitive action required when it comes to safety. Therefore, "appears" is the most accurately reflective of the regulation's intent to safeguard against potential risks associated with intoxicated individuals boarding an aircraft.