Feed water supplied to a flash type distilling plant will flash to vapor in the first-stage due to __________.

Correct Answer: Option D — flash chamber pressure being lower than the saturation pressure corresponding to the feed water temperature

Flash evaporation occurs when heated liquid is introduced into a chamber where the absolute pressure is maintained below the saturation pressure corresponding to the liquid's temperature. This pressure differential causes the liquid to instantly boil or 'flash' into vapor, separating the water from dissolved salts and impurities.