Fluorine (F) has the greatest electronegativity among the given elements: H, Cl, O, and Na. Electronegativity is the ability of an atom to attract electrons towards itself. On the Pauling scale, fluorine has an electronegativity value of 4.0, which is the highest value among the elements in the periodic table.

Electronegativity generally increases from left to right across a period (row) and decreases from top to bottom within a group (column) in the periodic table. Fluorine is located at the top-right corner of the periodic table, which makes it the most electronegative element among the given options.