Final answer:
The crew member is at rest relative to the tugboat, while the barge, riverbank, and downstream current are not.
Step-by-step explanation:
In this situation, the tugboat is moving upstream in a river with a constant downstream current. When the crew member is walking on the tugboat, they are at rest relative to the tugboat itself. This is because they are on the same object, so their motion relative to the tugboat is zero.
The barge, on the other hand, is being pulled by the tugboat and is moving with the same velocity as the tugboat. Therefore, it is not at rest relative to the tugboat.
The riverbank is also not at rest relative to the tugboat, as it is stationary and not moving with the tugboat.
The downstream current, however, is moving in the opposite direction of the tugboat's motion. Therefore, the downstream current is at rest relative to the tugboat.