The correct answer is - b. chlorine gas combined with sodium from the rocks.
Most of the water on the Earth is a salty water. It can be found in the oceans, seas, lakes, and even underground waters. The reason for the salinity of the waters is there's an abundance of chlorine and sodium. The chlorine gas easily interacts with the sodium which can be found in the rocks. Once they combine, they create what we know as salt, or chemically NaCl. Because of the large amount of salt that is created by the combination of the chlorine and the sodium, the water that very easily mixes with it, becomes salty.