JEE Main · 2023 · 10 Apr 2023 · Shift 1 · d and f Block Elements

The standard $M^{3+}/M^{2+}$ potential measures how readily the +3 ion accepts an electron. For manganese it is about $+1.5$ V, because the stable half-filled $d^5$ configuration of manganese(II) strongly resists oxidation, whereas for iron it is only about $+0.77$ V; the iron value is therefore the smaller of the two, so the first statement is wrong. The higher oxidation states of the first transition series are indeed stabilised by the small, highly electronegative oxide ion, as in chromate and permanganate, so the second statement is correct. For chromium the $C{r}^{3+}/C{r}^{2+}$ potential is about $-0.4$ V, so chromium(II) is a strong reducing agent and can reduce the hydrogen ion of a dilute acid to hydrogen gas, making the third statement correct. Vanadium(II) has a $3d^3$ configuration with three unpaired electrons, giving a spin-only moment of $\sqrt{3(3+2)}=\sqrt{15}\approx 3.9$ BM, well below the quoted 4.4 to 5.2 BM, so the fourth statement is wrong. Only statements (B) and (C) are correct.