- The melting point of Mn is lower than Fe because Mn has a lower atomic number and a less stable crystal structure compared to Fe.
- The melting point of Tc is lower than Ru due to the differences in atomic structure and bonding.
- The melting point of Re is lower than Os because Os has a higher atomic number and stronger metallic bonding, leading to a higher melting point. Thus, the correct answer is \( {Mn}<{Fe}, \ {Tc}<{Ru} \ {and} \ {Re}<{Os} \).