Question

elimination
substitution
addition polymerization
condensation polymerization

Explanation:

Step1: Identify reactant and product

Reactant is ethene ($C_2H_4$) with a double - bond, product is polyethylene with a long chain of $-CH_2-$ units.

Step2: Recall reaction types

Elimination involves removal of a small molecule; substitution replaces one atom or group. Addition polymerization occurs when monomers with double - bonds open up their double - bonds and add together. Condensation polymerization involves the loss of a small molecule (like water) during polymerization.

Step3: Determine the reaction type

Here, ethene monomers open their double - bonds and add together to form a long - chain polymer without the loss of any small molecule. So it is addition polymerization.

Answer:

addition polymerization