Answer (c) is correct.
If conditions are hot, high and humid the air is less dense. This reduces thrust and means when you lose the critical engine on takeoff there is less yawing effect from asymmetric thrust than there would be in denser air.
Vmcg concerns the calibrated airspeed for minimum control on the ground (i.e. using primary aerodynamic controls only to correct the yawing tendency). We know that flying surfaces are more effective the faster air flows over them so, if the yaw is less pronounced, the required airspeed for counteracting it need not be so high.
As elevation and temperature increase (Hot High and Humid) Vmcg decreases.
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