What must differ between the atom of two different elements

The atoms of different elements differ in the number of protons. That is how the elements are ordered in the periodic table. The number of protons of an atom is its atomic number. The elements are placed in the periodic table in ascending order. For example, H has 1 proton (atomic number 1), Helium has 2 protons (atomic number 2), lithium has 3 protons (atomic number 3), berilium has 4 protons (atomic number 4), and so on. Do not doubt it all the atoms of a specific element have the same number of protons and the atoms of diferent elements will never have the same number of protons.

The answer should be c.