A compound can be defined as a substance which is made up of 2 or more different types of atoms or ions combined in a definite proportion by chemical bonds.
A. Water - It is made up of 2 hydrogen atoms and 1 oxygen atom joined by chemical bonds. Hence, it is a compound.
B. Iodine- It generally exists as I2. thus it is made up of only 1 type of atoms. ∴ It is not a compound.
C. Calcium- It is an element not a compound.
D. Air - Air has many components which are not chemically bonded to each other, thus air is a mixture and not a compound.
Hence, Only water is a compound among water, iodine, calcium and air.