The correct answer is D. According to Coast Guard regulations, when the machinery spaces on a tank vessel are protected by a fixed foam extinguishing system, an additional secondary means of extinguishing the fire, such as a portable foam applicator unit, is required. This secondary protection ensures that in case the fixed system fails or is not fully effective, there is an alternative method available to fight the fire in the machinery spaces. The other options (A, B, and C) do not accurately describe the additional protection required in this scenario, as per the Coast Guard regulations.