This question says in straight and level flight at a
constant CAS.
A change in temperature will affect both the TAS and the local sound of speed
in the same ratio and mach number is TAS divided by
LSS so Mn will remain constant.
If you are climbing or descending then you must draw the RTM lines and CAS is
the same as RAS. Here you can ignore IAS because the difference between IAS and
RAS/CAS is a constant with height, it only varies with
speed as the flow around the airframe changes.