Q.Which of the following is the most reactive?

Dihydrogen exists as a diatomic molecule, $H_2$, rather than as free atoms, because a lone hydrogen atom has a single unpaired electron and is thermodynamically unstable; two such atoms readily share their electrons to form a strong covalent bond, attaining a lower-energy, filled duplet configuration. This H–H single bond has an unusually high bond dissociation enthalpy of about $435.88\,kJ\,mol^{-1}$, among the strongest single bonds known. This high bond enthalpy makes dihydrogen kinetically inert at room temperature — even though most of its reactions (with oxygen, halogens, nitrogen, metal oxides) are thermodynamically favourable and strongly exothermic, they proceed only slowly unless the molecule is first activated by heat, light, a catalyst, or an electric discharge to break the strong H–H bond. Once activated, $H_2$ reacts with metals to give hydrides, with non-metals such as $Cl_2$ to give $HCl$, with $O_2$ to give water, an …