121. In accordance with 33 CFR Subchapter O (Pollution), which of the following documents would be the most useful in dealing with a vessel oil spill resulting in a discharge into navigable waters?

The shipboard oil pollution emergency plan is the appropriate document for responding to a vessel oil spill resulting in a discharge into navigable waters, as it provides procedures and contact information for immediate response actions required by 33 CFR Subchapter O.