The answer is B. element.
An element is composed of only one kind of atom and cannot be separated into simpler substances. Oxygen (O) is the element.
A compound is a substance composed of two or more different atoms chemically bonded to one another, for example, water (H₂O) consists of 2 atoms of hydrogen (H) and 1 atom of oxygen (O), so it is the compound.
A mixture consists of two or more substances that are not chemically combined. Solutions and colloids are mixtures.