A、0.5mol/L NaCl溶液中Na+浓度为0.5mol/L×1=0.5mol/L;B、0.3mol/L Na2SO4溶液Na+浓度为0.3mol/L×2=0.6mol/L;C、0.4mol/L Na2CO3溶液中Na+浓度为0.4mol/L×2=0.8mol/L;D、0.15mol/L Na3PO4溶液中Na+浓度为0.15mol/L×3=0.45mol/L;故选C.