Answer is: C. Atomic number.
Atomic number (Z) is the number of protons (p⁺), which is characteristic of a chemical element.
For example, boron (B) is an element with atomic number 5, which means it has five protons.
Another example, silver (Ag) is an element with atomic number 47, which means it hs 47 protons.
First element on the periodic table is hydrogen (H) with atomic number 1, after hydrogen is helium (He) with atomic number 2, that lithium (Li) with atomic number 3 and so on until atomic number 118.
