Assertion: The ionic radii of O2- and Mg2+ are same. Reason: Both O2- and Mg2+ are isoelectronic species.

🧠 Isoelectronic does NOT mean equal in size — different proton counts make a big difference $\ce{O^{2-}}$ and $\ce{Mg^{2+}}$ both have 10 electrons (oxygen 8 + 2 = 10; magnesium 12 − 2 = 10). So they are isoelectronic. But the proton count is what decides size, and they differ here: O has 8 protons, Mg has 12. With 12 protons gripping the same 10 electrons, $\ce{Mg^{2+}}$ is squeezed much smaller than $\ce{O^{2-}}$.

🗺️ Evaluate Assertion and Reason Assertion: "$\ce{O^{2-}}$ and $\ce{Mg^{2+}}$ have the same ionic radii." Standard values: $\ce{O^{2-}} \approx 140\,pm$, $\ce{Mg^{2+}} \approx 72\,pm$. Almost a factor of 2 apart. False.

Reason: "Both are isoelectronic species." 10 electrons each. True.

So A is false, R is true.

⚠️ The trap Many students hear "isoelectronic" and assume "identical in every way". The word literally means same electron count — nothing more. Among isoelectronic species, the one with more protons is always smaller. So $\ce{Mg^{2+}}$ is smaller than $\ce{O^{2-}}$, and they are nowhere near the same size.

$\boxed{\text{Answer: (d)}}$