Answer:
Potassium (K)
Explanation:
Out of the four options, Potassium is the only one to fill a total of 4 energy levels.
Both Cesium (Cs) and Rubidium (Rb) fill more than three energy levels, with Cesium filling 6 and Rubidium filling 5.
Sodium (Na) only fills 3 energy levels.
Thus, that leaves Potassium (K), which has 4 energy levels.
Hope this helped ^-^
