Question

what do all of the elements in a given horizontal row of the periodic table have in common?
a they have the same number of electrons.
b they have the same chemical reactivity.
c they have the same total number of occupied sublevels.
d they have the same number of occupied principal energy levels.

Explanation:

Elements in a horizontal row (period) of the periodic table have the same number of occupied principal energy levels. For example, all elements in period 2 have electrons in 2 principal energy levels. The number of electrons varies across a period as atomic number increases. Chemical reactivity also changes across a period. And the number of occupied sublevels is not the same across a period.

Answer:

d. They have the same number of occupied principal energy levels.