Final answer:
In Flow Designer, an SMS text message is sent to users or groups through a notification. Notifications configure and dispatch messages in response to specific workflow events.
Step-by-step explanation:
To send an SMS text message to specified users or groups in Flow Designer, you would use d. A notification. Within the Flow Designer environment, notifications are how you can configure and send out messages, including SMS text messages, based upon specific events or conditions within a workflow. Actions are generally used to execute operations, conditions help to determine the path of the flow, and triggers initiate the workflow, but it is the notification that sends out the message.