The correct option is B element, compound
Elements are made of only one kind of atoms while compounds are made up of two or more kinds of atoms, chemically combined, in a definite ratio.
Oxygen molecule (O2) is made up of the same type of oxygen atoms. Hence, oxygen is an element. On the other hand, carbon dioxide (CO2) is made up of one atom of carbon and two atoms of oxygen. Hence, carbon dioxide is a compound.