The correct answer is D) flash chamber pressure being lower than the saturation pressure corresponding to the feed water temperature. In a flash-type distilling plant, the feed water is heated to a temperature higher than its normal boiling point. When this heated feed water enters the first-stage flash chamber, the lower pressure within the chamber causes the water to "flash" or suddenly vaporize. This occurs because the chamber pressure is lower than the saturation pressure corresponding to the feed water's temperature, allowing the water to rapidly convert to steam. The other options are incorrect because they do not accurately describe the primary mechanism that causes the feed water to flash to vapor in the first-stage of a flash-type distilling plant. Temperature, heat exchange surfaces, and atomization are not the direct causes of the flashing process, which is driven by the difference between the chamber pressure and the saturation pressure of the heated feed water.