The ionization enthalpy is the energy required to remove an electron from an atom or ion. In general, the ionization enthalpy tends to increase as you move across a period in the periodic table from left to right due to increasing nuclear charge and ionization enthalpy generally decreases as you move down the group. Therefore, in the case of fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At), the ionization enthalpy generally decreases as you move down the group.
Arranging them in decreasing order of ionization enthalpy:
F > Cl > Br > I > At
Fluorine has the highest ionization enthalpy among the given elements because it is the smallest atom and has the strongest nuclear charge, which holds its electrons more tightly. As you move down the group, the atomic size increases and the ionization enthalpy decreases since the outermost electrons are farther from the nucleus and experience weaker attractive forces.