Final answer:
Derek Shepherd cheated on Addison with Meredith Grey.
Step-by-step explanation:
The subject of your question is related to the TV show 'Grey's Anatomy', which falls under the category of entertainment and not academic subjects like Math, History, English, Biology, Chemistry, Physics, Social Studies, AP, SAT, Geography, Health, Arts, Business, Computers and Technology, French, German, Spanish, World Languages, Medicine, Law, or Engineering. The show focuses on the lives of surgical residents and attending physicians at Grey Sloan Memorial Hospital.
In the show, Derek Shepherd cheated on his wife, Addison Montgomery, with a fellow surgical resident named Meredith Grey, who later becomes his love interest and eventually his wife. So the correct answer to your question is A) Meredith Grey.