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'
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GMAT 1: 750 (Q50; V40; IR 8; AWA 5.5)

Debrief: https://gmatclub.com/forum/first-attempt-750-q50-v40-ir-8-awa-245721.html