Idea: The strongest oxidizing agent is the species that is most easily reduced.

So we compare the reduction tendencies of: $Mn^{3+},\; Fe^{3+},\; Ti^{3+},\; Cr^{3+}$

Each acts as an oxidizing agent by gaining electrons: $M^{3+} + e^- \rightarrow M^{2+}$

Use standard reduction potentials for the couples $M^{3+}/M^{2+}$:

• $Mn^{3+} + e^- \rightarrow Mn^{2+}, \quad E^\circ = +1.51\,V$
• $Fe^{3+} + e^- \rightarrow Fe^{2+}, \quad E^\circ = +0.77\,V$
• $Cr^{3+} + e^- \rightarrow Cr^{2+}, \quad E^\circ = -0.41\,V$
• $Ti^{3+} + e^- \rightarrow Ti^{2+}, \quad E^\circ \approx -0.37\,V$

Criterion: Larger, more positive $E^\circ$ means greater tendency to get reduced, hence stronger oxidizing agent.

Comparison: $Mn^{3+} > Fe^{3+} > Ti^{3+}, Cr^{3+}$ in oxidizing strength.

Conclusion: The strongest oxidizing agent is $\boxed{Mn^{3+}}$

Option check:

• A: $Mn^{3+}$ — Correct
• B: $Fe^{3+}$ — Weaker than $Mn^{3+}$
• C: $Ti^{3+}$ — Not strong oxidizing agent
• D: $Cr^{3+}$ — Not strong oxidizing agent