Answer: (Assuming we’re going by degrees Fahrenheit) I’d say 140.
Explanation: -40 would be beyond freezing point for water which is 32 degrees Fahrenheit and 40 would nearly be freezing point. For water to transform into its gas phase, the temperature would need to be greater. Therefore, 140 degrees Fahrenheit makes the most sense.
Although I did search it up and 212 degrees Fahrenheit (100 degrees Celsius) is the proven temperature at which water transforms into water vapor so... not sure if there’s more to the question or I’m interpreting the question wrong.