Answer: Option A.
Deep concavity on the mesial surface of the tooth
Step-by-step explanation:
The maxillary first premolar has different morphology with two roots and two canals. The most common anatomic feature that complicates periodontal maintenance in maxillary first premolar is Deep concavity on the mesial surface of the tooth because the The mesial surface of the crown shows a concavity near the cementoenamel junction and this concavity stretch on the mesial surface cervically and it's link a deep into the root trunk surface (referred to as the canine fossa)