Final answer:
In the TV show 'Grey's Anatomy,' Addison cheats on Derek with Mark Sloan.
Step-by-step explanation:
In the TV show 'Grey's Anatomy,' Addison does not cheat on Derek with any of the options listed (Mark Sloan, Pete Wilder, Sam Bennett, or Kevin Nelson). Addison does cheat on Derek with Mark Sloan, but he is not given as an option. Mark Sloan was a recurring character on 'Grey's Anatomy' who had an affair with Addison while she was still married to Derek.