The correct answer is C) Long enough to achieve at least one complete change of air. According to 46 CFR Subchapter T, which covers small passenger vessels, the regulations state that exhaust blowers in enclosed spaces containing gasoline-powered machinery must be operated long enough to achieve at least one complete change of air in the space before the engine is started. This is to ensure that any potentially explosive gasoline vapors are cleared from the space before ignition. The other options are incorrect because A) does not specify the required air changes, B) is too vague, and D) requires two air changes, which is more stringent than the regulatory requirement of at least one complete air change.