In which condition are first officers restricted from flying?

First officers are restricted from flying if they have less than 100 hours in the aircraft type. This guideline is put in place to ensure that pilots have sufficient familiarity and experience with a specific aircraft before taking on responsibilities in the cockpit. A minimum flight hour requirement is critical for safety, as it helps to ensure that pilots are skilled in handling the aircraft's systems and emergency situations.

The other factors listed do not represent valid restrictions on flight operations. Runway conditions and weather clarity are not bases for limiting a pilot's ability to fly, and age is not a factor in the qualifications necessary for flying as a first officer. Therefore, the requirement for a minimum of 100 hours in the aircraft type is the key regulatory measure aimed at maintaining safe operational standards within the flight crew.