Which of the following species is a free radical (odd-electron species) taking part directly in the catalytic destruction of stratospheric ozone?

Count valence electrons: ClO has 7 + 6 = 13 (odd), an odd-electron radical that reacts with O atoms to regenerate Cl. $\mathrm{ClONO}{}_2$ (7+5+6+6+6 = 30), $\mathrm{HOCl}$ (1+6+7 = 14) and $\mathrm{HCl}$ (8) all have even numbers of electrons and are stable, non-radical molecules.