What does calibrated airspeed (CAS) correct for?

Calibrated airspeed (CAS) is a measurement that corrects indicated airspeed (IAS) for instrument errors and also compensates for environmental factors such as air density. Instruments can have inherent inaccuracies due to design and manufacturing tolerances, and CAS seeks to provide a more accurate representation of an aircraft's performance by addressing those discrepancies.

Additionally, CAS takes into account how changes in temperature and pressure affect air density, which can influence the performance of the aircraft. By considering these factors, CAS provides a better indication of the aircraft's true airspeed relative to the surrounding air, making it essential for various flight operations, such as navigation, performance calculations, and safety assessments.

The other options do not encompass the full scope of what CAS accounts for; they either limit the corrections to altitude, instrument errors, or pressure alone without considering the combined effects of these variables.