A magnetic moment of 5.92 BM means there are 5 unpaired electrons because

Magnetic Moment = √ n(n+2)

There are four ligands attached to Mn²⁺ so the geometry has to be either square planar(dsp²) or tetrahedral(sp³). It can’t be square planar because all the five d orbitals in Mn²⁺ are singly filled and not vacant. So, this means the geometry is tetrahedral with 5 unpaired electrons giving a magnetic moment of 5.92 BM.