Final answer:
The elements among the options provided are O2 (oxygen), Na (sodium), and Cu (copper) because they consist of only one type of atom. NaCl (sodium chloride) and CaCO3 (calcium carbonate) are compounds, not elements.
Step-by-step explanation:
The question asks which of the following are elements: a) O2, b) Na, c) NaCl, d) Cu, e) CaCO3. An element is a substance that consists of only one type of atom and cannot be broken down into simpler substances by chemical means.
Among the options given: Na (sodium) and Cu (copper) are the pure elements as they consist of single types of atoms. Oxygen (O2) is a diatomic molecule, meaning it's comprised of two atoms of the same element joined together, so it also qualifies as an element in this context. On the other hand, NaCl (sodium chloride) and CaCO3 (calcium carbonate) are chemical compounds consisting of two or more elements chemically bonded together and are not pure elements. Therefore, the elements from the presented options are: a) O2, b) Na, and d) Cu.