Step 1: Understanding addition polymers.
Addition polymers are formed by the repeated addition of monomer units without the elimination of any small molecule.
Step 2: Analysis of given polymers.
Orlon is polyacrylonitrile and is formed by addition polymerization of acrylonitrile monomers.
Nylon-6,6, polyester, and urea formaldehyde polymer are condensation polymers formed with elimination of small molecules like water.
Step 3: Conclusion.
Therefore, the correct answer is (C) Orlon.