What altitude is understood to have all temperatures as negative in an FB?

The correct answer is 24,000 feet, as this altitude is commonly recognized in aviation meteorology as the level at which temperatures are expected to be negative in a standard atmosphere. This altitude lies within the stratosphere, where temperature inversion typically occurs, and where air temperature decreases with altitude in the lower atmosphere (troposphere) up to approximately 24,000 feet. Above this level, particularly in the stratosphere, temperatures begin to stabilize or can even increase due to the influence of ozone and other atmospheric conditions.

At altitudes below this point, depending on weather systems and other factors, temperatures might still reach above freezing. Therefore, the choice of 24,000 feet accurately reflects the average conditions under standard atmospheric models, making it the recognized altitude where all temperatures are generally negative.