When must pilots designate an alternate airport on all IFR flight plans?

When pilots create an IFR flight plan, they are required to designate an alternate airport under specific circumstances to ensure safety in the event that the primary destination cannot be landed upon. The requirement is primarily based on the forecasted weather conditions upon arrival and the approach minimums at the destination airport.

In this context, when the lowest compatible approach minimums at the destination airport are above 1,500 feet or 3 statute miles visibility, pilots must designate an alternate airport. This is because the higher minimums indicate a greater chance that the flight may be unable to land at the intended destination due to inadequate visibility or ceiling. Identifying an alternate ensures that the pilots have a viable option, should conditions at the primary airport be unsuitable for landing upon arrival.

The other options either do not fully encompass all the requirements or conditions under which an alternate is necessary. Thus, while alternate airports are required based on various weather considerations and regulations, option A correctly outlines a specific scenario that triggers the need for an alternate airport in IFR flight planning.