Water is known as the 'universal solvent' because:

water is capable of dissolving every substance.

water can dissolve many ionic and covalent compounds.

water can dissolve many substances, but often requires large amounts of heat to be added.

water is usually available.

The best answer is the second statement. It is considered as a universal solvent since most substances dissolve in water. And comparing it to other liquids, it is the one who dissolves a wide range of substances. Also, it is an important substance for every living thing.

a rain shadow is a dry region of land on the side that is protected against the prevailing winds of a mountain range. the prevailing winds are the winds that occur most of the time in a particular place of the earth

prevailing winds carry air toward the mountain range. as the air rises up over a mountain range, the air cools, water vapor condenses, and clouds form. on this side of the mountains, called the windward side, precipitation falls in the form of  rain  or snow. this dry air produces a  rain shadow.

a corroding iron, if that's one of your choices