Answer: C) Evaporation
You should boil a pot of salt water. As the water evaporates, it leaves the salt behind. The water molecules go into the air to make water vapor. The more water in the air, the more humid the air will be. The salt particles won't go into the air and won't evaporate. They simply are left behind in the pot. Since water evaporates best when its surface makes contact with the air, you should get a pot that has the largest surface area to speed up the evaporation process.
You could also place spoonfuls of water onto a piece of construction paper to spread out the water over a larger area. Place the paper on a dish to collect the water that seeps through (though the water will eventually evaporate). Placing the dish in the sunlight speeds up the process. Though keep in mind that evaporation is a slow process in general.
