Polymerisation of ethene uses addition reactions

Steps:

• Identify ethene as an alkene with a C=C double bond that can open for chain formation.
• Recall polymerisation joins monomers into polymers; for ethene, it forms polyethene.
• Determine the reaction type: no small molecules are lost, so it's addition, not condensation.
• Confirm by the general equation: n CH₂=CH₂ → (-CH₂-CH₂-)ₙ.

Why A is correct:

• Addition reactions involve alkenes like ethene adding across the double bond to form polymers without by-products, as in the initiation by free radicals.

Why the others are wrong:

• B: Condensation involves eliminating water or similar from monomers like diols and dicarboxylic acids.
• C: Cracking thermally decomposes large alkanes into smaller alkenes and alkanes.
• D: Esterification reacts carboxylic acids with alcohols to form esters and water.

Final answer: A