We see that a molecule unites with itself to form a long chain of the same molecule, this is called polymerization.
Now, an addition polymerization is when the whole molecule is part of the polymer and a condensation polymerization refers to when a part of the molecule is lost and is not part of the polymer.
In this case, the carbon double bond is broken to form the polymer, both CH2=CH2 molecules will be part of the polymer, so this is an addition polymerization reaction.
Answer: Addition polymerization
