1. The problem asks which pair of elements has the most similar chemical properties. 2. Chemical properties are most similar for elements in the same group (vertical column) of the periodic table because they have the same number of valence electrons. 3. Let's analyze each pair: - a) Na (Sodium) and Mg (Magnesium): Na is in group 1, Mg is in group 2, so different groups. - b) O (Oxygen) and S (Sulfur): Both are in group 16, so same group. - c) C (Carbon) and Ne (Neon): C is in group 14, Ne is in group 18, different groups. - d) Na (Sodium) and Cl (Chlorine): Na is group 1, Cl is group 17, different groups. 4. Since O and S are in the same group, they have the most similar chemical properties. Final answer: b) O and S