The correct assumption used to calculate the steady-state concentration of a pollutant in the lake, so that the concentration in the lake is equal to the one in the exit stream is that the pollutant is conservative, meaning that it does not react or transform chemically in the lake or in the exit stream. This assumption allows for the use of mass balance equations to calculate the steady-state concentration of the pollutant based on the inflow and outflow rates, assuming that there is no accumulation of the pollutant in the lake. The other assumptions listed (completely mixed lake, steady-state conditions, first-order reaction kinetics) may also be relevant in certain cases, but the conservative pollutant assumption is the most fundamental and necessary one for the calculation of steady-state pollutant concentrations.