Final answer:
The appropriate communication channel for addressing the employee's behavior is an in-person meeting.
Step-by-step explanation:
The appropriate communication channel for addressing the behavior of the employee who is regularly showing up late and dressed sloppily is in-person meeting. An in-person meeting allows for a face-to-face conversation where the manager can address the issue directly, provide feedback, and demonstrate the importance of punctuality and professionalism. It also allows for non-verbal cues to be observed and a more personal connection to be established between the manager and the employee.