Frontier Flight Attendants should ___________ the amount of alcohol being consumed on board.

The correct answer is to monitor the amount of alcohol being consumed on board. This responsibility is crucial for flight attendants as it directly affects the safety and comfort of all passengers during the flight. Monitoring helps ensure that passengers do not become over-intoxicated, which can lead to disruptive behavior and potentially jeopardize the safety of the flight.

By keeping an eye on alcohol consumption, flight attendants can intervene if necessary, such as by cutting off service to someone who has had enough or assessing if a passenger may need assistance. This duty is significant not only for compliant adherence to regulations but also for maintaining a positive onboard atmosphere.

The other options do not encapsulate the proactive role that flight attendants play in this context. While documenting may refer to recording incidents or behaviors, it does not imply active engagement with passenger behavior. Informing could suggest telling passengers about rules regarding alcohol, but it lacks the ongoing observation aspect. Encouraging alcohol consumption is contrary to safety protocols and would not be appropriate. Thus, monitoring is the most aligned option with the responsibilities of flight attendants as outlined in federal aviation regulations.