The unique character of polychaete annelids, such as bristle worms and fan worms, is c. parapodia.
Explanation: Polychaetes are distinguished from other annelids by their parapodia, which are paired, fleshy protrusions on each segment of their bodies. These structures serve various functions, including locomotion and respiration. Unlike clitellum, which is associated primarily with oligochaetes (another class of annelids), and cilia, which are present in various animal groups for movement and feeding, parapodia are specific to polychaetes. Therefore, when identifying a distinctive feature of polychaetes, parapodia stands out as their defining characteristic.