The species that has a spin only magnetic moment of 5.9 BM, is -

$\mathrm{Ni}(\mathrm{CO})_{4}\left(\mathrm{~T}_{\mathrm{d}}\right)$

$\left[\mathrm{MnBr}_{4}\right]^{2}-\left(\mathrm{T}_{\mathrm{d}}\right)$

$\left[\mathrm{NiCl}_{4}\right]^{2}-\left(\mathrm{T}_{\mathrm{d}}\right)$

$\left[\mathrm{Ni}(\mathrm{CN})_{4}\right]^{2-}$ (square planar)