Final answer:
The index Z in the formula ZX represents the atomic number or the number of protons in the nucleus of an atom. In this case, Z represents zinc.
Step-by-step explanation:
The unknown chemical element is represented by the formula ZX. The index Z in this formula represents the atomic number or the number of protons in the nucleus of an atom of the element. It is different for each element and defines the identity of the element.
For example, if the formula is ZnX, the element is zinc. Therefore, the correct answer to your question is 1) Zinc.