Water ballast placed in a tank that has been crude oil washed, but not water rinsed, shall be regarded as __________.

Correct Answer: Option C — dirty ballast

Under MARPOL Annex I, Crude Oil Washing (COW) removes heavy waxy residues with solvent action, but leaves residual oil films on tank surfaces. Therefore, ballast water loaded into a COW-cleaned tank that has not been subsequently water-rinsed is classified as 'dirty ballast' and must be transferred to slop tanks or shore reception facilities.