Which of the following can be a source of error in Spectrophotometry? Mark the wrong answer a) Differences in solvent
b) Acid base equilibrium (differences of pH)
c) Parasitic or stray radiation
d) Monochromatic radiation
I have some doubts about pH in spectrophotometry. I'm not sure if it is correct but here is what I thought:
a)Differences in solvent. It can be a source of error because disociation, reaction or asociation of the analyte with solvent can generate absorption products different of the analyte.
b)Acid base equilibrium. I don't know why pH is a variable to take into account in spectrophotometry. I thought maybe the case of the determination of phenolphthalein that  depends of pH medium. Spectrophotometry doesn´t work when phenolphtalein is in acidic medium. So pH is a multiplicative interference in this case.
c)Parasitic radiation. It is a source of error because  it is radiation that doesn't come from the analyte and it generates signal.
d)Monochromatic radiation. I think that is the wrong answer because it is what we need to avoid big changes in the absorbance as a function of the wavelength.
It is a very open ended question, but first three choices will cause errors. Regarding your pH issue, you are right, it is multiplicative error. Take the example of phenolphthalein, as you say that its color (=absorption spectrum) is different in acidic or basic medium. So pH change can shift the lambda max of analytes, hence the cause of error.
Phenolphthalein still absorbs light in the acidic medium. I cannot find phenolphthalein spectra, but let us take phenol red indicator, you can see drastic changes in absorbances at the lambda max as a function of pH.
If by some random chance, you were measuring at the isosbestic point, then pH will not affect absorbance but in all other cases pH will change the A values drastically.