Final answer:
The least reactive elements on the periodic table are the Noble Gases in group 18 due to their full valence electron shells, which make them chemically inert.
Step-by-step explanation:
The least reactive elements of the periodic table are the Noble Gases. These gases, which include helium, neon, argon, krypton, xenon, and radon, are found in group 18 and are characterized by their lack of chemical reactivity. This inertness is due to their full valence electron shells, which make them stable and unlikely to gain or lose electrons. In contrast, the alkali metals in group 1 and alkaline earth metals in group 2 are highly reactive, especially with water, due to their one and two valence electrons, respectively. Moreover, transition metals possess varying reactivities but are generally less reactive than groups 1 and 2 metals due to their partially filled d-subshells. Therefore, among the options provided, Noble Gases are the least reactive elements on the periodic table.