Under the Graduated Licensing System in Ontario, the supervising driver must not let their Blood Alcohol Concentration exceed:

Answer: C — 0.05 g%

This answer is correct because under the Graduated Licensing System in Ontario, supervising drivers must maintain a Blood Alcohol Concentration of 0.05 g% or lower. Option A, 0.00 g%, is incorrect because it implies that any amount of alcohol is not allowed, whereas in reality a small amount of alcohol is permitted as long as it falls within the legal limit. Option B, 0.08 g%, is also incorrect because this is the legal limit for fully licensed drivers, not supervising drivers under the Graduated Licensing System. Option D, 0.80 g%, is far too high and would result in severe impairment and possible legal consequences. Therefore, option C is the only appropriate and accurate response.