What cruising altitude should IFR aircraft fly at in uncontrolled airspace between FL180 and FL290 when headed from 180-359 degrees?

When flying in uncontrolled airspace at altitudes between FL180 and FL290, IFR aircraft are required to adhere to the "cruising altitude" rules established for air traffic separation. For aircraft on routes that are oriented from 180 to 359 degrees, they should be assigned even flight levels, such as FL200, FL220, or FL240.

This rule helps maintain vertical separation between aircraft traveling in opposite directions, as those flying from 0 to 179 degrees (the opposing direction) will utilize odd flight levels. Therefore, using even flight levels for aircraft heading between 180 and 359 degrees maximizes separation, reducing the risk of mid-air collisions and ensuring orderly traffic flow.

The options that suggest odd flight levels or specific altitude intervals would not comply with the established protocols for altitude assignments in this range and direction of flight. Choosing a specific altitude like FL210 also does not provide the necessary flexibility needed for different altitudes within the even flight level category, making it less applicable in this scenario.