The correct answer is A. True.
Self-phase modulation (SPM) is a nonlinear optical effect in which the refractive index of a material is modified by the intensity of the light propagating through it. This can cause the pulse spectrum to be modified, as the different frequencies of light will experience different refractive indices.
SPM can be a desirable effect, as it can be used to generate new frequencies of light or to compress pulses. However, it can also be a undesirable effect, as it can cause pulse broadening and distortion.
The amount of SPM that occurs depends on the material properties, the intensity of the light, and the length of the material. SPM can be mitigated by using materials with a low nonlinear refractive index, by using short pulses, or by using a technique called chirped pulse amplification.
In conclusion, self-phase modulation causes modifications to the pulse spectrum. This can be a desirable or undesirable effect, depending on the application.