"Sweating the rails" phenomenon consists in:

The question is about the phenomenon of "rail sweating" (pocenie się szyn). It is moisture settling on cooled rails, mainly in the autumn - answer B.

Steel rails cool down faster than the air at night. When warmer, humid air comes in the morning, water vapour condenses on the cold steel, just like on a glass taken out of the fridge. A thin layer of moisture forms on the rails and reduces the grip of the wheels, even if it has not rained. So it is not about the rails getting wet from rain (answer A) or water evaporating after rain (answer C). On autumn mornings you move off gently and brake earlier.

Legal basis: Art. 19 para. 1 of the Road Traffic Act (Prawo o ruchu drogowym)