$\begin{aligned}
& \text { (A) }\left[\mathbf{M n B r}_4\right]^{2-} \
& \mathrm{Mn}^{+2} \Rightarrow[\mathrm{Ar}] 3 \mathrm{~d}^5
\end{aligned}$ In presence of ligand field 
⇒sp3 hybridization, paramagnetic in nature (B) [FeF6]3− Fe+3⇒[Ar]3 d5 In presence of ligand field
⇒sp3 d2 hybridization, paramagnetic in nature (C) [Co(C2O4)3]3− Co+3⇒[Ar]3 d6 In presence of ligand field 
⇒d2sp3 hybridization, diamagnetic in nature (D) [Ni(CO)4] Ni0⇒[Ar]3 d84 s2 In presence of ligand field 
⇒sp3 hybridization, diamagnetic in nature