The sentence that is best written is...
b. The conveyor broke down, halting packaging.
In this case you have a clause that is the result of the action that happened in the first place. It is nicely separated and easily the best answer available to you in this example.
The next best answer would have been A, although it is somewhat difficult to see if the conveyor breaking down actually caused the packaging to halt. In the correct answer above, it is easy to see this construction.
