Most common epicenter of a radicular cyst is:: (A) Distal surface of tooth, (B) Apex of tooth involved, (C) Mesial surface of tooth, (D) Deep periodontal pocket
Answer is B. In most cases, the epicenter of a radicular cyst is located approximately at the apex of a nonvital tooth. It occasionally appears on the mesial or distal surface of a tooth root at the opening of an accessory canal or infrequently in a deep periodontal pocket. 
Oral radiology White and Pharaoh; 7th ed. Page no 335