Step 1: Understanding the Concept:
Addition polymers form when unsaturated monomers add to each other with no loss of small molecules. Condensation polymers form with loss of a small molecule such as water.
Step 2: Check each:
Nylon 6: from caprolactam (ring opening, with water as the initiator), classed as a condensation polymer.
Dacron: from ethylene glycol and terephthalic acid, a polyester formed by condensation.
Polythene: from \(\text{CH}_2=\text{CH}_2\) by addition. Correct.
Nylon 6,6: from hexamethylenediamine and adipic acid, a condensation polymer.
Final Answer:
Polythene is the addition polymer, option (C).
\[ \boxed{\text{Polythene}} \]