Regarding the chemistry of manganese and chromium oxoanions, the correct statement(s) is(are)

(A) Manganate(VI), $d^1$, is stable only in strongly alkaline solution. In acid it disproportionates: $3\mathrm{MnO_4^{2-}}+4\mathrm{H^+}\to2\mathrm{MnO_4^-}+\mathrm{MnO_2}+2\mathrm{H_2O}$; Mn goes from +6 to +7 and +4, so (A) is true. (B) The equilibrium $\mathrm{Cr_2O_7^{2-}+2OH^-\rightleftharpoons2CrO_4^{2-}+H_2O}$ keeps Cr at +6 on both sides, so it is an acid-base process, not redox; false. (C) Mn(VII) is $d^0$, so no $d$-$d$ transitions are possible; the intense purple colour is due to ligand-to-metal charge transfer from oxygen to manganese; false. (D) $\mathrm{Mn(VI)}$ is $d^1$ with one unpaired electron, so $\mu=\sqrt{1\cdot3}=1.73$ BM, not zero; false. Only A is correct.