How do I stop my rear derailleur from rubbing against my chassis?
To clarify, these Shimano Tourney derailleurs have their spring-loaded pivot at a different place to where they attach to the derailleur hangar. Pivot B is not a pivot at all, and shouldn't be free to move. However, it is currently attached at the wrong angle. The angle made between the hangar and the rigid part of the derailleur should be more acute, as in this image.



Note that there is a protruding tab on the derailleur's attachment point, and on the hangar itself. These tabs should be pressed against each other, such that the arrangement prevents that part of the derailleur from rotating in an anticlockwise direction relative to the hangar. I've highlighted the tabs that need to mate together in the image below.



You may need to fine tune the chain length and derailleur adjustment further, follow the Shimano service instuctions for RD-TX55.