I would have to say that falling snow would attenuate an HF signal but to what extent I couldn't say. A heavy fall of large flakes of wet snow would be a lot worse than that fine dusty stuff.
How much a ground wave is attenuated depends on the conductivity of the ground it's travelling over. The more conductive the ground, the less the attenuation. Saline water is quite conductive, granite is almost an insulator, pure water is an insulator. You couldn't possibly say what effect snow on the ground had, there's too many variables.
The conspiracy theorist in me suspects that JAA have planted these type of questions because they don't like people scoring 100% in their tests.
In reality you either have to second guess what the examiner wants or better still find the correct answer and learn it by rote. Not ideal but it is one of the failing of multiple choice exams.
In my PPL Nav there is a question that asks "how is VDF information displayed to ATC?". Apparently "radial lines on a CRT" is correct, "a digital display" is wrong and "not at all because the VDF equipment is broken and there's no money in the budget to fix it" is not listed at all....