Electronegativities of the elements na, al, p, and cl follo a specific trend across the period .based on this trend the atoms of which electron will be most strongly attracted to ?

Cl. The Cl atom will most strongly attract the bonding electrons to itself.

Electronegativities increase from left to right in the Periodic Table.

All the elements are in Row 3, so the order of electronegativities is:

Na < Al <P <Cl

hydrocarbons are organic compounds that contain only hydrogens and carbons. they can be saturated (those which contain only carbon-carbon single bonds) or unsaturated (those which contain carbon-carbon double or triple bonds) in nature. hydrocarbons can undergo several reactions like substitution, elimination etc.  

on substituting one or more hydrogen atoms in hydrocarbon it results in the formation of haloalkane. as the halogen atoms are large compared to the carbon and hydrogen atoms, due to which the molecular weight increases and the bond become polar because of presence electronegative halogen atom and results in the increase in boiling point of the haloalkane.

thus, the correct option is (b) that is the boiling point of the new compound increases on substituting a hydrogen atom with a halogen in a hydrocarbon.