Under what circumstances must an operator select two alternate destinations for an IFR flight?

The correct answer is that an operator must select two alternate destinations for an IFR (Instrument Flight Rules) flight specifically when weather conditions are below planning minima. This is crucial because alternate airports serve as safety measures in case the destination airport's conditions deteriorate, making landing impossible.

The Federal Aviation Administration (FAA) regulations specify that alternate airports must be designated when the weather at the intended destination is forecast to be below certain minima at the time of arrival. This requirement ensures that the pilot has a backup plan should the weather not allow for a safe landing at the intended airport.

Selecting two alternates is particularly critical under these conditions to provide more options for safe landing. The other choices do not necessarily mandate the selection of two alternate destinations. While it's wise to consider flight duration and the availability of weather information in flight planning, those factors alone do not trigger the specific requirement for two alternates unless tied specifically to weather minima considerations. Therefore, option B directly addresses the necessary condition for selecting two alternates.