The correct answer is D) To overcome the osmotic pressure generated by the difference in solution concentrations. In a reverse-osmosis freshwater generator, the high pressure pump is required to operate in the 750 to 1200 psig range to overcome the osmotic pressure created by the difference in salt concentrations between the feed seawater and the freshwater being produced. The osmotic pressure, which opposes the flow of water across the semi-permeable membrane, can be as high as 1000 psig for seawater. The high pressure pump generates enough force to push the seawater through the membrane, allowing the freshwater to be extracted while leaving the salts and minerals behind. The other answer choices are incorrect because the high pressure is not primarily needed to force the seawater through pre-filters or pre-treatment devices, but rather to overcome the osmotic pressure differential across the semi-permeable membrane.