How to solve & answer ?
Across groups of s -block elements melting and boiling points decrease down the group as for small size elements there is stronger metallic bonding which gives rise to higher mp/bp.
(Across groups no of unpaired electrons involving in bonding is same so couldn’t compare in that fashion)
S-block elements have comparatively weaker metallic bonds so low mp/bp compared to d block elements.
Across the period (in s-block)size decreases as well as valence shell electrons increases ,so by this logic along period mp/bp increases.
Now coming to d-block elements .
D-block elements have comparatively larger mp/bp with s block elements as they have higher no of valence electrons for metallic bonding.
Across groups in d-block mp/bp increases down the group.In ncert I haven’t found any logical explanation for this but it is given as a conclusion from the graphs.(Check the paragraph another generalisation.....)
Conclusion:Mp/bp depends on the effectiveness of the metallic bonding in metals.
It is due to involvement of valence electrons plus size factor(generally)(But exception arises)
If Iam wrong in anything ..pls do correct me
There is a exception in d-block elements (in the pic pls do refer the graphs for clarity)
Here is one more graph with indication of elements.