Command and prohibition

The blue-circle / red-circle distinction that trips people up.

Command signs

Blue means command. You must turn right at the next road.
A blue disc with 10 T is a command. Only vehicles with a gross vehicle mass over 10 tons may use this road. Anything lighter may not drive there.
Remember, this is going to be a command sign. That means that you have to obey the sign at all times, whether it's nighttime, whether it's the morning, or whether visibility conditions are going to be bad, right? So, this sign is telling you that you must switch on your headlights. Therefore, option C is going to be our correct answer.
Again, this is going to be a command sign, therefore you have to obey the sign at all times. So, this sign is going to indicate to you that you have to switch off your headlights, right? Irrespective of the time of day or visibility. Therefore, the correct answer would be option A, switch off your headlights immediately and continue driving without them while in the indicated area.

A red circle prohibits. Vehicles longer than 15 m may not go past this sign.
Prohibition. Vehicles higher than 4.42 m may not drive past. Watch for low bridges.
The sign that they are going to be describing, that's going to be a prohibitory sign. That means that no vehicle may enter that road from that specific direction. Therefore, option C would be correct.