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'