The correct answer is C) from the open deck. The regulations for tank vessels carrying grades A, B, C or D liquid cargoes require that access to the cargo pump room must be from the open deck. This isolates the cargo pump room from other areas of the vessel that may contain potential ignition sources, as per the safety requirements for these hazardous liquid cargoes. The other options are incorrect because: A) the distance requirement is not relevant for access, B) power ventilation is not the only requirement for access, and D) isolating the pump room from ignition sources is achieved by accessing it from the open deck, not necessarily any other specific location.