Which character(s) are sent by the shore station to indicate it has shifted control to, and is awaiting instructions from, the vessel?

Correct Answer: Option B — GA+?

The 'GA+?' (Go Ahead) signal is transmitted by the coast station to indicate that it has successfully established the connection and is now ready to receive instructions from the vessel. It serves as a prompt for the operator to input the next command, such as a routing code or message text.