To maintain plasma osmolality -> Need to exactly replace all the fluids lost in the day
She cannot concentrate urine above 450 mOsm/kg , so the minimum amount of water required to be excreted by kidneys is 1 (to excrete the 450 mOsm she accumulates per day). The minimum excretion water required is necessarily at max concentration; if you were to say produce diluter urine, say 225mOsm/kg, this would require 2 L of water. The question wants the minimum possible water volume, so we assume she’s concentrating to the max.
1 L losses from kidney + 900 mL insensible + 100 mL in sweat and feces = 2L losses -> need to ingest 2 L of water to replace.