Final answer:
Neon (Ne) is the least reactive element listed because it is a noble gas with a complete valence shell, which makes it chemically inert.
So, the correct answer is A.
Step-by-step explanation:
The least reactive element among Neon (Ne), Magnesium (Mg), Lithium (Li), and Chlorine (Cl) is Neon (Ne). Neon is a noble gas and is inert because it has a full valence shell of electrons, which makes it non-reactive under normal conditions. Noble gases are known for their lack of chemical reactivity due to their complete electron shells.
So, the correct answer is A.