Answer:
C its atomic number
Step-by-step explanation:
Every element has a unique atomic number. The atomic number is always the same for a given element and does not change.
The number of protons in an atom is the atomic number. Protons are located in the nucleus. Being nuclei particles, they do not have a variable number. Their number is fixed and we can easily identify an atom with the number of protons, which is the atomic number.