Question

the periodic table organizes elements according to increasing a atomic mass. b energy level c atomic number. d valence number.

Explanation:

The periodic table organizes elements by increasing atomic number (the number of protons in an atom's nucleus). Atomic mass was used in early versions but modern periodic tables use atomic number. Energy level relates to electron shells, and valence number is about outer electrons, neither are the basis for periodic table organization.

Answer:

C. atomic number