The correct option is C selenium
A polyatomic element is a chemical element that naturally exists as a compound molecule containing more than three atoms of the same element. There are only three polyatomic elements in the periodic table, namely- selenium, sulphur and phosphorous. Sodium is a monoatomic (contains only one atom) element. Nitrogen is a diatomic (contains two atoms) element. Carbon is monoatomic (contains only one atom) element.