As the bond dissociation energy decreases reducing nature of the substance increases because the bond can be broken down easily and can donate electron easily and make the other substance to be reduced.So the bond dissociation energy is inversely proportional to reducing nature.The order of bond dissociation energy of given molecules is NH_3>PH_3>AsH_3>SbH_3 due to increase in size of the central atom.So the strongest reducing agent is SbH_3 .Hence option D is correct.