Mathematicians joke that a coffee mug is indistinguishable

Mathematicians joke that a coffee mug is indistinguishable from a bagel, referring to the fact that these two items are topologically equivalent.

There's nothing wrong with the original sentence. We can also eliminate the other options as follows.

a. joke that a coffee mug is indistinguishable from a bagel, referring to the fact that these two items are topologically equivalent Correct Option
b. referring to their topological equivalence, joke that a coffee mug is indistinguishable from a bagel. The reference of the pronoun their is not clear. 'Their' can refer to mathematicians or to coffee mug and bagel
c. have a joke according to which a coffee mug is indistinguishable for its topological equivalence to a bagel. Indistinguishable should always be followed by 'from'. Hence, usage of indistinguishable is incorrect.
d. who are referring to their topological equivalence joke that a coffee mug is indistinguishable from a bagel. The pronoun their refers to mathematicians and not to coffee mug and bagel
e. joke that a coffee mug is indistinguishable from a bagel, which refers to the fact that these two items are topologically equivalent Usage of which is incorrect as here, which is referring to 'bagel'