The correct statement (s) about Cr²⁺ and Mn³⁺ is / are  [atomic number of Cr = 24 and Mn =25]

(a). In aqueous solution Cr²⁺ (3d⁴) acts as a reducing agent, oxidising itself to Cr³⁺(3d³) that  gives a completely half-filled t_2g level in octahedral ligand field of H₂O.

(b). Mn³⁺(3d⁴) is an oxidising agent as it is reduced to Mn²⁺(3d⁵), a completely half-filled stable configuration.

(c). Both Cr²⁺ and Mn³⁺ have d⁴ configuration.

(d). 3d⁴Cr²⁺ and $3d^{4}C{r}^{2+}\left(aq\right)\stackrel{R.A}{\to }C{r}^{3+}\left(aq\right)+e^{-}$

Hence (d) is wrong statement.