Among these, it is water which is a compound.
A compound is a substance which is made up of two or more different elements. Apart from this, the different elements in a compound are bound by a chemical bond in a fixed proportion. Water is a compound which is made up of two different elements which are hydrogen and oxygen. The chemical formula of water is H2O. Also, the proportion of hydrogen to oxygen in water is definite which is 2:1. Among other options, iodine and calcium are elements. As far as air is concerned, it is a mixture of various gases not a compound. So, correct option is water.
