Which is a compound.

A. Water B. Iodine c. Calcium

D. Air

C. Is the answer. Water is a compound

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.