The correct option is A Fe3+
For the strongest oxidizing agent, the oxidizing potential should be least. Here, the oxidizing potential of Fe+2 is less than that of [Fe(CN)6]4−. Therefore, Fe+2 is stronger oxidizing agent than [Fe(CN)6]4−. Also, the stronger oxidizing agent should easily reduce itself. Here, Fe+3 is easily reduced than Fe+2. Therefore, among all the four, Fe+3 is the stronger oxidizing agent.