What is the lowest usable flight level for an altimeter setting of 29.92 or higher?

The lowest usable flight level for an altimeter setting of 29.92 inches of mercury or higher is FL180. This standard is set for aircraft operating under Instrument Flight Rules (IFR) to ensure safe vertical separation between aircraft. When the altimeter setting is at or above 29.92, pilots are instructed to begin their cruise altitude at FL180, which is 18,000 feet above mean sea level. This altitude serves as a transition point where aircraft are expected to operate using flight levels, enhancing safety and minimizing the risk of altitude-related conflicts.

Flight levels are used to represent altitudes at and above 18,000 feet, and FL180 is significant because it is the first flight level above the geographical area where the altimeter transitions from local settings to standardized pressure. This ensures that all aircraft are consistently referencing the same pressure level for altitude, enabling safe separation and communication among air traffic control and pilots.

Choosing any of the other options would not provide the correct answer because they represent altitudes above FL180. For example, FL190 and FL200 are above this transition altitude, which means they are not applicable when determining the lowest usable flight level at the referenced altimeter setting.