Which one of the following is the most fundamental characteristic is an element?

The atomic number (Z) is the most fundamental characteristic of an element because it represents the number of protons in the nucleus, which uniquely defines an element's identity. While properties like melting point and colour can vary or be shared, no two different elements have the same atomic number. Historically, elements were organized by atomic weight, but modern chemistry recognizes that the atomic number determines the electron configuration in a neutral atom, which in turn governs the element's chemical behavior and position on the periodic table. Even isotopes of the same element, which differ in atomic weight due to varying neutron counts, maintain the same atomic number, confirming that the proton count is the constant, defining feature. Therefore, the atomic number is the primary factor that distinguishes one chemical element from another.