A、4L 0.5mol/LNaCl溶液中Na+浓度为0.5mol/L×1=0.5mol/L；B、1L 0.3mol/LNa2SO4溶液Na+浓度为0.3mol/L×2=0.6mol/L；C、5L 0.4mol/LNaOH溶液中Na+浓度为0.4mol/L×1=0.4mol/L；D、2L0.15mol/LNa3PO4溶液中Na+浓度为0.15mol/L×3=0.45mol/L；故选B．
