What property do the following elements have in common?
li, c, and f

They all belong in the same period, thus they have the same energy levels which is 2.

In the periodic table, elements are categorized by group and period. Periods are arranged in rows. Below is a picture of the periodic table and where these elements are.

As you can see, they all belong in the same period (row). Elements in the same period have the same number of atomic orbitals, or in other words, they have the same electron energy level. The period number tells you the energy level. Lithium (Li), Carbon (C), and Fluorine (F) are in period 2, so they all have two energy levels.