Incidence of C shaped canal is maximum for:
Some C-shaped canals are difficult to interpret on radiographs and often are not identified until an endodontic access is made. 
These are commonly seen in mandibular second molars and maxillary first molars, especially when roots of these teeth appear very close or fused.
Mandibular 1st molar
Mandibular 2nd molar
Maxillary 1st premolar
Maxillary 1st molar