The correct answer is C) shift cargo aft. When a vessel is loaded and down slightly by the head, shifting cargo aft will help to adjust the trim and bring the vessel more upright in the water. This is because shifting weight aft will raise the bow and lower the stern, correcting the down-by-the-head condition. Adding ballast aft or loading more cargo aft would have a similar effect, but shifting the cargo is the most direct way to adjust the trim without adding more weight. The other options, while potentially effective, are less preferable than simply shifting the existing cargo. Adding ballast or loading more cargo could affect the vessel's stability and overall weight, which may not be necessary if the trim can be corrected by repositioning the cargo.