Answer
D) atomic number.
Step-by-step explanation
Periodicity is caused by regular and predictable variations in element atomic structure. Mendeleev organized elements according to recurring properties to make a periodic table of elements.
In the modern periodic table, the elements are listed in order of increasing atomic number. The atomic number is the number of protons in the nucleus of an atom. The number of protons defines the identity of an element.
Hence, the chemical elements display periodicity when they're arranged according to atomic number.
The correct answer is option D) atomic number.