which of the following periodic properties has no unit?

a- ionization enthalpy

b- atomic radius

c- electronegativity

d- electron gain enthalpy

Electronegativity has no units. It is the tendency of an element to attract the shared pair of electrons in a covalent bond towards it. It is described on some scales like Pauling scale which has no units.
Atomic radius has a unit of length and Ionisation enthalpy and electron gain enthalpy are described in units of kJ/mol.