Assertion: Aryl halides cannot be prepared by replacement of hydroxyl group of phenol by halogen atom. 

Reason: Phenol reacts with halogen acids violently. 

Assertion is correct but the Reason is incorrect.

Aryl halides cannot be prepared by replacing hydroxyl group of phenols because the C-O bond in phenol has a partial double bond character and is difficult to break being stronger than a single bond.