(A) Convection
Step-by-step explanation:
When molecules of a fluid are heated, they bounce against each other and the sides of the bathroom and move farther apart. When the heated molecules in the steam have more space between them, the steam is less dense and lighter than the heavier, cooler air in the room, so it rises. As the steam transfers some of its heat to the air around it, we can see the steam and air rise in a current. We also see rising steam when we take a hot shower. This transfer of heat energy to movement is called convection.
Similarly, when we open up the window of the bathroom, the steam from the hot water gets enough space to expand and being light in weight rises up and goes out through the window and avoids fogging.