According to industry practices, the freeze point of the remaining fluid film should be at least:

The correct response regarding the freeze point of the remaining fluid film aligns with industry practices that advocate for safety and performance in various weather conditions, particularly in aviation. Maintaining a freeze point that is at least 20 degrees Fahrenheit below the ambient temperature ensures that the de-icing or anti-icing fluid remains effective in preventing ice formation on the aircraft's surfaces during flight.

This standard provides a margin of safety that protects the aircraft from potential ice accumulation, which can adversely affect aerodynamic performance and safety. By having a freeze point significantly lower than the ambient temperature, operational safety is maintained even in unexpected weather fluctuations.

Other options, while potentially less conservative, do not provide the same level of assurance against freezing conditions. A freeze point of 10 degrees Fahrenheit below ambient temperature may be insufficient in extreme cold or variable weather, leading to risks associated with ice formation. Similarly, options indicating freeze points such as 5 degrees Celsius below zero or at 32 degrees Fahrenheit do not account for the necessary buffer to ensure that the fluid film remains unfrozen and effective during operation, thereby lacking the safety margin established by industry standards.