Final answer:
In the communication model, Jack is the receiver when he gets an email from Janice, and becomes the sender when he replies to confirm.
Step-by-step explanation:
Using the communication model, Jack in this scenario is the receiver. Janice, who sends the initial email asking if he can arrive at work early, acts as the sender. When Jack replies, affirming that he will come to work early, he is providing feedback to the sender, closing the loop of the communication process. The model showcases how messages are sent and received between individuals, often with the roles of sender and receiver swapping as the conversation progresses.