Bond dissociation energies of 

a) $\mathrm{HF}$  b)  $\mathrm{HCl}$  c)  $\mathrm{HBr}$   follow the order

There is a large difference between electronegativity of H and F, so the bond is stronger. As we go down the group, the difference between electronegativity decreases and therefore, bond dissociation energy also decreases. The order of bond dissociation energies are H−F>H−Cl>H−Br.