The most common compound on earth is:

water

nitrogen

phosphorus

oxygen

Answer: water

Explanation: A compound is a substance which contains more than one element chemically combined together in a fixed proportion. Nitrogen (N), phosphorus (P) and oxygen (O) are elements while water is a compound. Water has a chemical formula of H2O, containing two elements hydrogen and oxygen.

Water

Explanation:

A compound means a substance (something) that is made of 2 or more elements.

Elements are the substances found on the periodic table. If you can find something on the periodic table, it cannot be a compound.

Nitrogen is "N" (7th square).

Phosphorus is "P" (15th square).

Oxygen is "O" (8th square).

The compound "water" is H₂O. It uses the elements "H" (hydrogen) and "O" (oxygen).

Since water is the only compound in those choices, the answer has to be water.

Other facts!

The Earth's surface has mostly water. About two-thirds (2/3) of Earth's surface is water.

The Earth's atmosphere is mostly Nitrogen (78%).

We breathe oxygen (which is O₂).