The most frequent tooth to be impacted is :: (A) Lower third molar, (B) Lower canine, (C) Upper third molar, (D) Upper premolar
Answer is A. Impacted tooth :

Is the tooth that has failed to erupt completely or partially to its correct position in the dental arch and its eruption potential has been lost.

Order of frequency :

Mandibular 3rd molar (i.e. Lower 3rd molar).
Maxillary 3rd molar (Upper 3rd molar).
Maxillary canine.