It is important to conduct multiple trials because

It is important to conduct multiple trials so one can get the best results, additionally, multiple trials with result in minimal errors. Finally it is important to conduct multiple trials because it will show you the actual answer/solution/whatever. If you do something once their may of been a random unknown affect and you would of never knew because you only conducted the experiement once.

Explanation:

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.