Refer to figure.
IMPORTANT NOTE: The correct answer for this question has been recently updated following recent feedback. Students have confirmed that the option "The airflow becomes more turbulent due to the presence of areas of Increased compressibility slowing down the speed of the airflow." Was not correct in exams. Please let us know if you come across this question in your examination. Thank you!

As an aircraft accelerates to high speeds, it enters a regime where the effects of compressibility become significant. The critical Mach number (M_CRIT) is the airspeed at which the local airflow over certain parts of the aircraft (typically near the wing's leading edge or other protuberances) reaches the speed of sound (Mach 1).

The airflow over the aircraft's surfaces starts to approach or reach the speed of sound. This means that some parts of the aircraft may experience local airflow velocities at or near Mach 1. This increased compressibility is what leads to changes in the behavior of the airflow.

As the airflow reaches or exceeds Mach 1 locally, shock waves can start to form. These are sudden, discontinuous changes in air pressure and density. Shock waves can form on the wings, fuselage, and other parts of the aircraft that have particular shapes or characteristics that cause local acceleration of the airflow.

One of the consequences of compressibility effects is that the total pressure of the airflow increases. This increase in total pressure can lead to an artificially high reading on instruments like the airspeed indicator. CAS measures the pressure differential between the pitot tube (which senses ram air pressure) and static pressure. At high speeds, the ram air pressure component becomes significant, causing CAS to read higher than TAS.