IV normal saline will increase hydrostatic pressure in the vasculature. This isotonic solution is freely filtered across the capillaries, which is collected by lymphatics and can be picked up in this experiment.
Experiment is asking which would increase pulmonary lymph flow, which I interpreted as which of these increases blood flow/intravascular volume. Endothelin, phenylephrine, low o2 concentrations all cause vasoconstriction. Co2 unlike in other organs (i.e. brain, muscle) does not cause vasodilation in the lungs so i left this one to the side. IV saline will for sure increase intravascular volume and blood flow so I leaned more toward this. The other explanation made a good point about the albumin solution in that it may cause reabsorption due to a high oncotic pressure (i.e. with albumin)