The correct answer is C) areas where direct impingement from deck mounted machines cannot be satisfactorily reached. Bottom mounted crude oil washing machines are primarily used in areas of the tank where the deck-mounted washing machines cannot effectively clean, such as the bottom and lower sides of the tank. The bottom-mounted machines allow for direct impingement of the washing fluid onto these hard-to-reach areas, ensuring thorough cleaning compared to the limitations of the deck-mounted machines. The other options are incorrect because: A) the bottom-mounted machines are not used to wash the entire bottom of the tank, just the areas inaccessible to the deck-mounted machines; B) they are not used for the majority of crude oil tank cleaning, but rather a supplementary system; and D) they are not used for all vertical surfaces, as the deck-mounted machines can effectively clean most vertical areas.