The critical Mach number (Mcrit) is the:

Free-stream Mach number at which airflow first reaches Mach 1.0 somewhere on the aircraft

Correct

Maximum operating Mach number

Speed of sound at sea level

Mach number for best range

Explanation

Mcrit is the flight Mach number at which the local airflow first accelerates to sonic speed (M=1.0) at some point on the airframe.

Test yourself with random questions from Principles of Flight and all other ATPL subjects.