ANS:D - all 'a', 'b' & 'c'.

The correct statement regarding the dew point of an air-water vapor mixture is: Dew point of an air-water vapor mixture corresponds to any point on the humidity chart is obtained by projecting a line through this point parallel to the temperature axis and to the saturation curve. This statement accurately describes how to determine the dew point on a humidity chart. When plotting a point on a humidity chart representing the conditions of a moist air mixture (specifically, the dry-bulb temperature and relative humidity), the dew point can be found by projecting a line from that point parallel to the temperature axis until it intersects the saturation curve. The temperature at this intersection point is the dew point temperature. The other statements are not correct:

• Decreases with decrease in pressure: The dew point of an air-water vapor mixture generally increases with a decrease in pressure. This is because lowering the pressure reduces the saturation vapor pressure, making it easier for water vapor to condense and reach its dew point.
• At constant humidity & total pressure is fixed: The dew point of an air-water vapor mixture is influenced by changes in both humidity and pressure. Keeping the total pressure fixed while changing other conditions (such as temperature or humidity) can alter the dew point.
• All 'a', 'b' & 'c': As explained above, statements (a) and (b) are not correct, so this option cannot be correct.