Given below are two statements : one is labelled as Assertion and the other is labelled as Reason.
Assertion: The ionic radii of $$O^{2-}$$ and $$Mg^{2+}$$ are same.
Reason: Both $$O^{2-}$$ and $$Mg^{2+}$$ are isoelectronic species.

We need to evaluate the Assertion-Reason statement about $$O^{2-}$$ and $$Mg^{2+}$$.

First, we check whether they are isoelectronic. $$O^{2-}$$ has 10 electrons (8 + 2), and $$Mg^{2+}$$ also has 10 electrons (12 - 2), so the Reason is true.

Next, we examine whether their ionic radii are the same. Since both species have 10 electrons but different nuclear charges (Z = 8 for $$O^{2-}$$ and Z = 12 for $$Mg^{2+}$$), the one with the greater nuclear charge pulls the electron cloud more tightly and has a smaller ionic radius. Therefore, $$r(O^{2-}) > r(Mg^{2+})$$, and the Assertion is false.

In conclusion, the Assertion is false but the Reason is true.

The correct answer is Option D.