Under what conditions must pilots designate an alternate airport on all IFR flight plans?

Pilots must designate an alternate airport on all IFR flight plans in several specific situations to ensure safety and compliance with regulations. When the lowest compatible approach minimums for the point of intended landing are greater than a ceiling of 1,500 feet or visibility of 3 statute miles, there is a significant risk that the destination airport could be below the minimums for landing upon arrival, making an alternate necessary.

Additionally, if the required navigational aids (NAVAIDs) for the approach to the destination airport are unmonitored, it raises a concern regarding the reliability of the navigation information that the pilots are using, further necessitating the need for an alternate airport.

Lastly, when the destination airport lacks weather reporting capability, it becomes challenging to determine whether weather conditions are suitable for landing, which could again lead to a scenario where having an alternate is crucial.

In essence, these conditions ensure that pilots have a backup plan for safe landings, which is vital for managing changing weather and navigational uncertainties along the route. Therefore, an alternate must be filed under all the stated conditions, making the comprehensive scenario the correct answer.