Answer
D is strongest reducing agent while A is the weakest reducing agent.
Work Step by Step
Because for the last reaction where $D^{3+}$ is reduced to D, the reaction has a negative reduction potential $E^0$ (-1.59)therefore the reduction reaction is non-spontaneous while reverse reaction (i.e. oxidation) is spontaneous. Due to feasible oxidation, D produces $D^{3+}$ and electrons and reduce other substances easily.
On the other hand, in case of first reaction reduction potential $E^0$ is positive(1.33V). In this case reduction reaction is spontaneous thus reverse reaction(i.e. oxidation) is not feasible. As oxidation is not feasible, this reaction cannot produce an electron and reduce other substances. So A is the weakest reducing agent.
