Final answer:
The maximum air temperature for which the condition can be met is 0 °C. The reasonableness of this condition depends on various factors related to aircraft design and operation.
Step-by-step explanation:
The maximum air temperature for which the given condition can be met can be determined using the freezing point of water. Since visible moisture is present, this indicates the presence of water. According to the statement, the nacelle anti-ice system should be operated when the temperature is below a certain value in degrees Celsius. For the condition to be met, the temperature should be below 0 °C, which is the freezing point of water. Therefore, the maximum air temperature for which this condition can be met is 0 °C.
As for whether this is a reasonable condition to impose on the design, it depends on the specific requirements and considerations of the aircraft design and operation. The condition is intended to prevent ice from forming on the nacelle, which could affect the performance and safety of the aircraft. However, there may be other factors that need to be taken into account, such as the availability of anti-ice systems and the expected operating conditions of the aircraft.