1. Concept used

   The strongest oxidising agent is the species which gets reduced most easily.

   In electrochemistry, this corresponds to the largest positive standard reduction potential $E^\circ$.

2. Given reduction potentials

   • A: $\mathrm{Sn^{4+} + 2e^- \rightarrow Sn^{2+}}$, $E^\circ = +1.15\,\text{V}$
   • B: $\mathrm{Al^{3+} + 3e^- \rightarrow Al}$, $E^\circ = -1.66\,\text{V}$
   • C: $\mathrm{Pb^{4+} + 2e^- \rightarrow Pb^{2+}}$, $E^\circ = +1.67\,\text{V}$
   • D: $\mathrm{Tl^{3+} + 3e^- \rightarrow Tl}$, $E^\circ = +1.26\,\text{V}$

3. Compare the values

   The values are: $+1.15,\;-1.66,\;+1.67,\;+1.26$

   The highest reduction potential is: $+1.67\,\text{V}$

4. Inference

   Therefore, the species with the greatest tendency to get reduced, and hence the strongest oxidising capacity, is: $\mathrm{Pb^{4+}}$

5. Option check

   • A: weaker than C
   • B: negative potential, so not a strong oxidising agent
   • C: maximum positive $E^\circ$ → strongest oxidising agent
   • D: positive, but less than C

Final Answer: C