Which of the following statements are true about metallic bonds?

a. they hold together ionic bonds.

b. they bond a metal to a nonmetal.

c. they bond identical atoms together.

d. they hold together atoms with different electronegative.

B. They bond identical atoms together

Explanation:

Metallic Bond:  This is the bond that exists among the atoms of the metal.  This is a type of electrostatic attraction between conduction electrons of metallic atom and their positively charged metal ions.

                                        (figure attached)

Example:  

Copper is a metallic element and its atoms consist of metallic bond among their selves.  

Options A, B, D are not correct as all states for a kind of ionic bond formation.

Ionic bond:  

It form by the complete transfer of electron from one atom to other

It is formed among two atoms of different electronegativities.

Also metals are electro-positive and non -metals are electro-negative so they form ionic bond.

Example is NaCl where sodium is a metal and Chloride is non-metal and they from ionic bond.

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.