The correct answer is: C. Ionization energy
Electronegativity is a measure of the tendency of an atom to attract electrons to itself. It is measured on a scale of 0 to 4, with fluorine having the highest electronegativity (4.0) and cesium having the lowest (0.7).
Electron affinity is the energy released when an electron is added to a neutral atom. It is also measured on a scale of 0 to 4, with fluorine having the highest electron affinity (3.44 eV) and cesium having the lowest
(0.79 eV).Ionization energy is the energy required to remove an electron from an atom in its gaseous state. It is measured in kilojoules per mole (kJ/mol).
Excitation potential is the energy required to excite an electron from its ground state to a higher energy state. It is measured in electron volts (eV).
Of the four options, only ionization energy has units.