C. The moving portal platform breaks the black platform and the whole thing goes through the portal.

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Jokes aside, B.

Devoid of any other external forces, in the cube's reference frame there's functionally no difference between it moving towards the portal or the portal moving towards it at a constant acceleration.

Ergo the cube should be pushed out the other side at the same momentum that the system had prior to the collision, and not be simply plopped out as in answer A.