I would say that Vmcg has to do with the effectiveness of your rudder to maintain directional control on takeoff. At high altitudes and temperatures the density of the air is low, that means that you need a higher TAS (same IAS !) to achieve the same directional effectiveness.
Also, at high altitudes the output power of your engine is lower which means that in the case of an engine failure the amount of yaw generated will be less so less rudder input is required to compensate.
Answer A is incorrect because at high altitudes versus low altitudes you fly at the same IAS for same amount of lift, only TAS changes with air density.
Answer B is incorrect because Vmcg will determine V1.
Answer D is incorrect because the IAS is not decreasing at constant TAS and decreasing air density (suppose unlimited engine power).
Correct answer (and only possible answer) is C.