Answer: C. She took the most beautiful photographs of Yosemite Park that I have ever seen.
Explanation: This is the correct answer on Edge 2021 also, I just took the practice quiz and made a 100%. Sentences A. and D are joined with a coordinating conjunction and B is joined with a semicolon. Therefore, they are compound sentences, but the question is asking which is NOT a compound sentence and that would be C as it does not contain any of these requirements for a compound sentence. Hope this helps ^-^.