The correct answer is A. The term "oily mixture" is defined in the Pollution Prevention Regulations as a mixture with any oil content. This includes bilge water, tank washings, and other oil-contaminated liquids. The regulations aim to prevent the discharge of these oily mixtures, which can harm the marine environment. The other answer choices are incorrect because they do not accurately reflect the regulatory definition of "oily mixture" under the Pollution Prevention Regulations.