Enthalpy of atomization is the energy required to break off atoms, which are joined together. From top to bottom in a group, the atomic radii increases due to addition of extra electron shells. Atoms with larger radii attract each other more weakly than atoms with smaller radii, so it is easier to separate the larger size atoms by the less input of energy and hence on moving down a group (s-block and p -block) have low atomization enthalpy. But in the case of d -block elements enthalpy of atomization increases on moving down a group due to presence of lanthanide and actinides contractions. Hence, correct option is (c).