Under the federal regulations of 33CFR Subchapter O, if a vessel equipped with a Type II Marine Sanitation Device enters a body of water where the discharge of treated or untreated sewage is prohibited, which of the following methods of securing the device to prevent the discharge of sewage is ACCEPTABLE if it is impractical to close the seacock and prevent its opening?

Correct Answer: Option D — Locking the door to the space enclosing the toilets with a padlock or door handle key lock to prevent entry when the vessel is entering a "no discharge zone".

33 CFR 159.7 requires that when a vessel is in a no-discharge zone, the Marine Sanitation Device must be secured to prevent overboard discharge. Locking the toilet space is an acceptable physical method to ensure the system cannot be operated, effectively preventing illegal sewage discharge in restricted waters.