1. Concept used

A reducing agent is the species that gets oxidized easily. So, the strongest reducing agent will be the species whose corresponding reduction potential is most negative.

If for a couple $\text{Ox} + ne^- \rightarrow \text{Red}$ $E^\circ$ is very negative, then the reverse reaction $\text{Red} \rightarrow \text{Ox} + ne^-$ is very favorable, meaning the reduced form is a strong reducing agent.

2. Given standard reduction potentials

$E^\circ_{\mathrm{Cr_2O_7^{2-}/Cr^{3+}}}=+1.33\,\text{V}$ $E^\circ_{\mathrm{Cl_2/Cl^-}}=+1.36\,\text{V}$ $E^\circ_{\mathrm{MnO_4^-/Mn^{2+}}}=+1.51\,\text{V}$ $E^\circ_{\mathrm{Cr^{3+}/Cr}}=-0.74\,\text{V}$

3. Identify which option can act as reducing agent

A reducing agent should be in the reduced form of a redox couple.

• Option A: $\mathrm{Cl^-}$ It is the reduced form in the couple $\mathrm{Cl_2 + 2e^- \rightarrow 2Cl^-} \qquad E^\circ=+1.36\,\text{V}$ Since reduction potential is highly positive, the reverse oxidation of $\mathrm{Cl^-}$ is not very favorable compared to stronger reducing agents.

• Option B: $\mathrm{MnO_4^-}$ This is already a highly oxidized species and acts as an oxidizing agent, not reducing agent.

• Option C: $\mathrm{Cr}$ From $\mathrm{Cr^{3+} + 3e^- \rightarrow Cr} \qquad E^\circ=-0.74\,\text{V}$ Since the reduction potential is negative, the reverse reaction $\mathrm{Cr \rightarrow Cr^{3+} + 3e^-}$ is favorable. Hence $\mathrm{Cr}$ is a strong reducing agent.

• Option D: $\mathrm{Mn^{2+}}$ It is the reduced form of permanganate: $\mathrm{MnO_4^- + 8H^+ + 5e^- \rightarrow Mn^{2+} + 4H_2O} \qquad E^\circ=+1.51\,\text{V}$ A very large positive reduction potential means $\mathrm{MnO_4^-}$ is a very strong oxidizing agent; correspondingly, $\mathrm{Mn^{2+}}$ is a very weak reducing agent.

4. Compare reducing strengths

Among the reduced forms listed:

• $\mathrm{Cl^-}$ corresponds to $E^\circ=+1.36\,\text{V}$
• $\mathrm{Mn^{2+}}$ corresponds to $E^\circ=+1.51\,\text{V}$
• $\mathrm{Cr}$ corresponds to $E^\circ=-0.74\,\text{V}$

The most negative reduction potential corresponds to the strongest reducing agent.

Therefore, $\boxed{\mathrm{Cr}}$

5. Final answer

The strongest reducing agent is: $\boxed{\text{C: }\mathrm{Cr}}$

6. Comparison with stored correct answer

Stored correct answer: C

My derived answer: C

So, they agree.