The correct options are
A Chemical reactions in which a pair of atoms or a group of atoms are removed from a molecule are called elimination reactions.
B Reactions in which the hydrogen atom of an alkane is substituted by an incoming group/ion are called substitution reactions.
D ∙Cl+∙Cl⟶Cl−Cl
is a possible termination step of a chlorination reaction
Theory :
Chemical reactions in which a pair of atoms or a group of atoms are removed from a molecule are called elimination reactions.
Reactions in which the hydrogen atom of an alkane is substituted by an incoming group/ion are called substitution reactions.
Reactions in which the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule are called rearrangement reactions. Often a substituent moves from one atom to another in the same molecule.
Chemical reactions in which an atom or a group of atoms is added to molecule are called addition reactions.
For a halogenation reaction :
In the initiation step , homolytic cleavage of halogen takes place forming a halogen free radical i.e. ∙X
This is because X−X bond is weaker as compared to C−H bond.
Posssible termination steps of a chlorination reaction :
∙Cl+∙Cl⟶Cl−Cl
∙CH3+∙CH3⟶CH3−CH3
∙Cl+∙CH3⟶CH3−Cl