Consider the following two molecules:
The two molecules above can be described as:
The molecule on the left has the chlorines in the cis configuration, while the one on the right has them arranged trans. Cis and trans are a type of diastereomer. Thus (B) is the right answer.
A: Enantiomers are mirror images of each other. The mirror image of a cis arrangement would still be cis, thus this is incorrect.
C: Tautomers are a form of constitutional isomer in which the bonding between the atoms is changed. The molecules depicted have the same bonding arrangment, so they are not tautomers. Also, by far the most common tautomer on the MCAT is the ketone / enol tautomer, which this is not.
D: Conformers can be freely interconverted by rotation around a single bond, with no bond-breaking. Here, there’s no way to convert between cis and trans without breaking bonds.