Introduction:
In this question, we are comparing the properties of two transition metal ions, Cr2+ and Mn3+, both of which have a d4 electron configuration. We need to determine whether they are reducing agents or oxidizing agents.

Explanation:
To understand the reducing and oxidizing properties of these ions, we need to consider their electron configurations and the trends in oxidation states of transition metals.

1. Electron Configuration:
Cr2+ has a d4 configuration, which means it has 4 electrons in its d orbital. The electron configuration of Cr2+ is [Ar] 3d4. Similarly, Mn3+ also has a d4 configuration, with the electron configuration of [Ar] 3d4.

2. Oxidation States:
Transition metals can exhibit multiple oxidation states due to the availability of d orbitals for electron transfer. The most common oxidation states for Cr and Mn are +2 and +3, respectively. In these oxidation states, both Cr and Mn have a d4 configuration.

3. Reducing Agents:
Reducing agents are substances that can donate electrons to another species, thereby getting oxidized themselves. In the given question, Mn3+ has a higher oxidation state (+3) compared to the most common oxidation state of Mn (+2). This means Mn3+ has a greater tendency to accept electrons and get reduced. Therefore, Mn3+ acts as a reducing agent.

4. Oxidizing Agents:
Oxidizing agents are substances that can accept electrons from another species, thereby getting reduced themselves. Cr2+ has a lower oxidation state (+2) compared to the most common oxidation state of Cr (+3). This indicates that Cr2+ has a greater tendency to donate electrons and get oxidized. Therefore, Cr2+ acts as an oxidizing agent.

Conclusion:
Based on the above analysis, we can conclude that Mn3+ is an oxidizing agent because it has a higher oxidation state (+3) compared to the most common oxidation state of Mn (+2). On the other hand, Cr2+ is a reducing agent because it has a lower oxidation state (+2) compared to the most common oxidation state of Cr (+3). Therefore, the correct answer is option 'D' - Mn3+ is an oxidizing agent while Cr2+ is a reducing agent.