Question

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

Explanation:

The modern periodic table is structured by arranging elements in order of their increasing atomic number (number of protons in the nucleus), which fixes inconsistencies that occurred when organizing by atomic mass. Energy levels, valence numbers, and atomic mass do not serve as the primary organizing principle for the modern periodic table.

Answer:

C. atomic number.