What is happening in a double replacement reaction

A double displacement reaction, also known as a double replacement reaction or metathesis reaction, is a type of reaction that occurs when the cations and anions switch between two reactants to form new products. ... A gas formation reaction is when two compounds react to form a gaseous product.

Explanation:

it would be b. 6co2 + 6h2o --> c6h12o6 + 6o2

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.