Which altitude does the standard temperature at sea level refer to?

The standard temperature at sea level is defined as 15 degrees Celsius at an altitude of 0 feet. This is a crucial reference point in aviation and meteorology, as it establishes a baseline for various calculations, including those related to air density and performance metrics for lighter-than-air vehicles.

Understanding this reference temperature is essential because it helps pilots and engineers assess how altitude affects temperature and, consequently, the performance of lighter-than-air crafts like balloons and airships. Air temperature typically decreases with altitude according to the standard atmosphere model, so knowing the sea level temperature allows for more accurate predictions of how these craft will behave at various elevations.

The other options present different temperatures at varying altitudes but do not adhere to the standard definition for sea level. Thus, they do not provide the correct baseline needed for this context.