National Dental Examining Board of Canada (NDEB) Practice Exam

For a Class I amalgam restoration, the occlusal cavosurface margin should be 90 degrees. This means that the cavity preparation should have a sharp and right angle with the rest of the tooth structure. A 45 degree margin would be too steep and could potentially weaken the structural integrity of the tooth, while a 60 degree margin would be less steep but still not ideal. Option D of having a beveled margin is also incorrect because beveling creates a sloped surface rather than a sharp angle, which can be a weak point for the restoration. The angle at the cavosurface margin is important for providing proper retention and resistance against occlusal forces, and a 90 degree margin is the most optimal for achieving this.