Final answer:
The action of ending a session as agreed while informing the client they can continue at the next session is known as Maintaining professional boundaries, which is essential for professional and scheduled interactions.
Step-by-step explanation:
End just as agreed, but tell the client he can continue at the next scheduled session. The correct option that defines this action is D. Maintaining professional boundaries. This involves ending the session on time as previously arranged with the client and not extending beyond the allocated timeframe. Provisioning for the client to continue in the next session supports the professional relationship while adhering to the schedule. It's important to note that options A, B, and C do not correctly describe the scenario as they imply terminating the session unexpectedly, scheduling sessions back-to-back without adequate breaks, or prolonging the current session, respectively.