On a day with heavy fog, as shown in the image, which fog lights should you activate if available?

Why this is the correct answer

In dense fog conditions like the one shown in the image, Spanish traffic rules make a clear distinction between front and rear fog lights: 1. REAR fog lights are MANDATORY: When visibility is especially poor (dense fog, very heavy rain, heavy snowfall), you are legally required to switch on your rear fog light(s). Their purpose is to make your vehicle clearly visible to drivers behind you, preventing dangerous rear-end collisions. 2. FRONT fog lights are OPTIONAL: The regulation states that in reduced visibility you must use either front fog lights OR dipped beam (low beam) OR main beam (high beam) headlights. Since you can choose dipped beam instead, front fog lights are not compulsory. Additionally, front fog lights are not mandatory equipment on all vehicles, so the law cannot require their use. This is why the correct answer is B: front fog lights are optional, rear fog lights are mandatory. If another question seemed to say front fog lights are also mandatory, it may have been referring to the general obligation to use some form of forward lighting (which can be satisfied by dipped beam headlights alone).