Final answer:
The treatment of a plasmid with type IB topoisomerase in high salt solution generally leads to increased linking number due to topoisomerase activity. Transferring the plasmid back to a low-salt solution will likely result in a decreased linking number as the enzyme reverses its action.
Step-by-step explanation:
When a relaxed plasmid undergoes a transition from B to Z conformation and is then treated with a type IB topoisomerase in a high salt solution, it experiences a change in its supercoiling state due to the activity of topoisomerase, which can relieve the pressure that results from supercoiling by breaking and reforming DNA's phosphate backbone as mentioned in sources related to DNA replication and topoisomerases. Transferring this plasmid back to a low salt solution would typically result in the topoisomerase reversing its activity, thus restoring the molecule's original linking number and supercoiling state. Therefore, the correct outcome would likely be b) a Decreased linking number, assuming that the treatment under high salt conditions has led to an increase in the linking number.