The correct answer is C) shift cargo aft. To trim a fully loaded vessel slightly by the stern, the best action is to shift cargo aft. This is because shifting cargo to the aft section of the vessel will lower the forward draft and raise the aft draft, resulting in the desired trim. The other options, such as loading more cargo aft (A) or shifting ballast (B, D), are less optimal as they may affect the vessel's stability and overall trim. Shifting cargo aft is the correct choice as it directly addresses the issue of the vessel being down slightly by the head, without significantly impacting the overall stability or loading conditions of the vessel.