Correct option is c) d = 2f
As we know that in case of Achromatism by separated doublet separation distances between two lenses is given by : d = (ω1f1+ ω2f2)/ (ω1+ω2)
In case when lenses are of same material, ω1 = ω2 = ω Then d = (f1+f2)/2 in this problem f1=3f & f2=f
d = (f1+f2)/2=3f+f/2=2f
Hygen’s Eyepiece satisfies the condition of Achromatism and has the same focal length for all colours. This makes Hygen’s Eyepiece apparently free from lateral chromatic aberration.