Which of the following is necessary for collisions to be successful?
a. enough kinetic energy and favorable geometry
b. enough kinetic energy only
c. sufficient potential energy only
d. favorable geometry only

A. enough kinetic energy and favorable geometry

Explanation: According to collision theory, the rate of a reaction depends upon the number of effective collisions which means the collisions which result in the formation of product which in turn depend on following two factors:-

a) Energy factor: In order for a collision to be successful, the colliding molecules must cross a energy barrier which is called as threshold energy. Thus molecules with energy less than threshold energy are not able to result into products.

b) Orientation factor: the colliding molecules must have proper orientations at the time of collision otherwise they do not result in product formation.

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.