The correct answer is a) Sulfur S.
Oxygen (O) and sulfur (S) are both elements found in Group 16 of the periodic table, also known as the chalcogens. They share similar chemical properties due to having the same number of valence electrons, which influences how they react with other elements.
Both oxygen and sulfur readily form compounds with metals and nonmetals, and they exhibit similar oxidation states, typically -2 in most of their compounds. This is in contrast to nitrogen (N) and hydrogen (H), which belong to different groups in the periodic table. Fluorine (F), while also a nonmetal, is actually in Group 17 and behaves quite differently compared to oxygen.
Therefore, when considering the chemical properties, sulfur is the most similar to oxygen among the options provided.