Which one of the following has the lowest melting point?

The correct answer is Option (2) → Mn

The high melting points of transition metals come from strong metallic bonding in which (n−1)d electrons also take part. In any row the melting points rise to a maximum near d⁵, but manganese is an anomaly: its stable half-filled 3d⁵4s² configuration holds the electrons firmly, fewer of them take part in metallic bonding, and Mn has a much lower melting point than its neighbours. Approximate melting points: Mn 1519 K, Ni 1728 K, Fe 1811 K, Cr 2180 K.

NCERT (2026-27 edition):
Physical Properties – “… In any row the melting points of these metals rise to a maximum at d⁵ except for anomalous values of Mn and Tc and fall regularly as the atomic number increases.”