Strong reducing and oxidizing agents among the following, respectively, are

Step 1 — Identify strong reducing agents (easily oxidised):

• $\mathrm{Eu^{2+}}$: Europium prefers the +3 state (stable $4f^7$ configuration at +3). $\mathrm{Eu^{2+}}$ ($4f^7$) is very stable but readily loses an electron to reach $\mathrm{Eu^{3+}}$ ($4f^6$)...

Actually, $\mathrm{Eu^{2+}}$ has $4f^7$ (half-filled, stable), but it is a strong reducing agent because it can donate electrons to reach $\mathrm{Eu^{3+}}$.

• $\mathrm{Ce^{3+}}$: Stable configuration; poor reducing agent.

Step 2 — Identify strong oxidizing agents (easily reduced):

• $\mathrm{Ce^{4+}}$: Ce prefers +3 state (empty $4f$ shell at +4 is less stable). $\mathrm{Ce^{4+}}$ readily accepts an electron to become $\mathrm{Ce^{3+}}$ (stable $4f^0$) → strong oxidizing agent ✓
• $\mathrm{Tb^{4+}}$: Less common; $\mathrm{Ce^{4+}}$ is the classic strong oxidizer among lanthanides.

Step 3 — Conclusion:

• Strong reducing agent: $\mathrm{Eu^{2+}}$ (donates $e^-$ easily)
• Strong oxidizing agent: $\mathrm{Ce^{4+}}$ (accepts $e^-$ easily)

Answer: Option (4) — $\mathrm{Eu^{2+}}$ and $\mathrm{Ce^{4+}}$