Final answer:
The best SD for a student with autism to come to an adult would be a personalized and clear request with context, like saying the student's name and explaining the reason for the request.
Step-by-step explanation:
The best SD (discriminative stimulus) to get a student with autism to come to an adult would be Option A: "Hi Johnny. Please come over here right now because it is time to start our lesson." This option is most effective because it provides a clear, direct, and personable cue with an explicit reason attached, thereby giving context to the request. It not only personalizes the interaction by addressing the student by name but also sets clear expectations by explaining the reason for the prompt. For a student with autism, the additional information can help them understand the importance of the request and what is expected of them next.