- The lenses in an optical microscope bend the light...to form an image ...that is much larger than the one (the image) actually viewed. (OK)
A: than it
B: than the one
C: one than
D: than one which
B is correct because
much larger than is being used to compare two nouns:
1.
an image of a specimen that is much larger than
2.
the (other) one i.e. the other image of the specimen
A is incorrect because "it" refers back to 'an image of a specimen'. That is, *it(self) is much larger than it(itself).
ungrammatical
C would be correct iff we added "a":
...
a much larger one than actually viewed.
D would be correct iff we added "the...is":
...much larger than
the one which is actually viewed.
:D