The element most chemically similar to sulfur (S) is Phosphorus (P).
Sulfur and phosphorus are both in the same group (Group 16) of the periodic table, which means they share similar chemical properties. Chlorine is in Group 17, sodium is in Group 1, and oxygen is in Group 16 as well but is in a different period than sulfur. Therefore, phosphorus is the best choice among the options given.