The correct answer is C. The number of protons in every atom of an element
Step-by-step explanation:
In chemistry, the atomic number of an atom is equivalent to the number of protons (particles with a positive electric charge) that can be found in the atoms of the element being analyzed. This implies, the atomic number of every element is always the same, but still, the number of neutrons (a particle with no charge) and electrons (particles with a negative electric charge) can be different in atoms. According to this, the factor that determines the atomic number is "the number of protons in every atom of an element".