The value of Mach does not relate to temperature directly.
Mach = TAS/LSS
LSS is affected by temperature, so as you climb the temperature drops, therefore the LSS drops. If you are in an isothermal layer then the LSS will remain constant as temperature remains constant.
However TAS is dependent on air density, which as you climb also drops whatever the air temperature is. Remember the formula :
0.5 rho V squared
When density decreases your TAS increases to maintain a constant CAS.
Therefore Mach depends on altitude, not temperature.
VMO relates to a Mach number. Answer A
Regards
NHF