The rear fog light must be used whenever...
A)driving on a mountain pass.
B)driving through a tunnel.
C)driving in heavy snowfall.Correct
# Explanation: Rear Fog Light
The rear fog light must be used specifically when there is **heavy snowfall**, since visibility is significantly reduced and it is necessary for other vehicles to see you from behind. It is not mandatory to use it on mountain passes or in tunnels, only when adverse weather conditions (fog, heavy rain or snow) require it to increase road safety.