Step 1: Understanding addition polymers.
Addition polymers are formed by the repeated addition of monomer units without the elimination of any small molecules.
Step 2: Analyzing the options.
Dacron, urea-formaldehyde polymer, and Nylon-6 are formed by condensation polymerization with elimination of small molecules.
Step 3: Identifying polythene.
Polythene is formed by the addition polymerization of ethene molecules without loss of any by-product.
Step 4: Conclusion.
Thus, polythene is an example of an addition polymer.