The correct answer is $\boxed{\text{C}}$.
Electronegativity is a measure of the tendency of an atom to attract electrons to itself. Atoms with a high electronegativity are more electronegative than atoms with a low electronegativity.
The electronegativity of an atom is determined by its number of valence electrons and its nuclear charge. Atoms with more valence electrons are more electronegative than atoms with fewer valence electrons. This is because valence electrons are located in the outermost shell of an atom and are therefore more easily attracted to the nucleus.
Atoms with a higher nuclear charge are also more electronegative than atoms with a lower nuclear charge. This is because the nucleus of an atom has a positive charge, which attracts electrons. Atoms with a higher nuclear charge have a stronger attraction for electrons than atoms with a lower nuclear charge.
In the given options, chlorine (Cl) has the highest electronegativity. This is because chlorine has 7 valence electrons and a high nuclear charge. Phosphorus (P), sulfur (S), and aluminum (Al) have 5, 6, and 3 valence electrons, respectively, and lower nuclear charges than chlorine. Therefore, chlorine is more electronegative than phosphorus, sulfur, and aluminum.