JEE Main · 2024 · 01 Feb 2024 · Shift 2 · d and f Block Elements

$C{r}^{2+}$ has configuration $3d^4$; losing one electron gives $C{r}^{3+}$ ($3d^3$), a stable half-filled $t_{2g}^3$ arrangement in octahedral fields, so $C{r}^{2+}$ readily loses an electron and acts as a reducing agent. $M{n}^{3+}$ has configuration $3d^4$; gaining one electron gives $M{n}^{2+}$ ($3d^5$), the extra-stable half-filled $d$-configuration, so $M{n}^{3+}$ readily accepts an electron and acts as an oxidising agent. Both behaviours are driven by the same drive toward a stable half-filled $d$-subshell, so Assertion and Reason are both true and Reason correctly explains Assertion.