When the external pressure is 450 kpa, what is the vapor pressure of water at its boiling point?

The boiling point is the point at which the vapor pressure becomes equal to the external pressure. In fact, below this point the vapor pressure is lower and the water bubbles can't escape because of the higher external pressure. Instead, when the water pressure reaches the external pressure, bubbles start to escape the water, and this is when boiling occurs. Therefore, the answer is "water pressure is 450 kPa, equal to the external pressure"

Independent variable is something that is changed in a experiment.