If a person views two red lights,but one is more obviously red than the other,what differs between the two lights

A)  purity of the light
B)  wavelength 
C)  brightness of the light
D)  wave amplitude

The purity of the light refers to how narrow or broad the range of wavelengths is in the light. The more pure the light, the narrower the range of wavelengths. If one red light appears more obviously red than the other, it is likely because it has a purer red color, which means a more narrow range of wavelengths. The difference in brightness (C) or wave amplitude (D) would not affect the perceived color difference between the two red lights.