Oxygen is a/an ______ and carbon dioxide is a/an ________.

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 $\left(O_2\right)$ is made up of the same type of oxygen atoms. Hence, oxygen is an element. On the other hand, carbon dioxide $\left(C{O}_2\right)$ is made up of one atom of carbon and two atoms of oxygen. Hence, carbon dioxide is a compound.