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.

the mantle

explanation:

earth is divided into three layers: the crust or the outermost layer on which we live; the underlying, middle layer - the mantle - and the core (the innermost layer of earth).