As per the games own physics it’s A.

Speedy thing goes in. Speedy thing comes out. It doesn’t matter if the portals are moving even in the game. The only thing that matters is the momentum of the object moving through the portals.

All y’all physics nerds can take your degrees and shove em cause it’s video game logic not real life.

The only reason the portal velocity isn’t considered in the game is because the portal velocity is always 0. Moving portals just isn’t something they programmed it to do. If they had, I imagine it would be B, if only because it creates more interesting gameplay options.