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a
The melting point of Mn is lower than that of Cr and Fe in 3d series
b
The melting point of Tc is higher than that of Mo but lower than that of Ru in 4d series.
c
The melting point of W (tungsten) is highest in 5d series.
d
The order of melting points of 3d, 4d and 5d series is: 5d >4d >3d
answer is B.
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Detailed Solution
The melting point of Tc is lower than that of Mo.The dip in melting point of Mn (3d series) and Tc (4d series) is because of their stable half filled configuration, i.e., (Mn = 3d5 4s2) and Tc (4d5 5s2). Hence e-'s are held tightly by the nucleus so that the delocalization is less and the metallic bond is much weaker than that of preceding and next elements.