If a published approach at an airfield is out of service, the worst weather for it to qualify as an alternate must be at least:

When an approach at an airfield is out of service, the criteria for determining whether a nearby airport can be utilized as an alternate depends on the minimum weather conditions required for alternate airports. The minimums for IFR alternate airports specifically take into account the weather that must be present at the alternate in relation to how it is classified when one or more approaches at the destination are unavailable.

For an airport with a published approach that is not available, the minimum visibility and ceiling requirements generally increase. The standard criteria dictate that if the approach is out of service, the alternate airport must have a ceiling of at least 600 feet and visibility of at least 2.25 miles to be designated as suitable. This requirement accounts for potential limitations in approach capabilities, ensuring that pilots have sufficient conditions for a safe approach and landing.

Thus, the choice indicating 600 foot ceiling and 2.25 mile visibility aligns with the regulatory requirements for alternate airports when the primary approach is not available, making it the correct answer.