Step 1: Understanding the Concept:
This question tests factual recall from the chapter "From the Diary of Anne Frank." It refers to a specific event mentioned by Anne in her diary.
Step 2: Detailed Explanation:
In her diary, Anne Frank recounts her family's life. She mentions that in the summer of 1941, her grandmother became sick and required surgery. The phrase used to describe this event is that her grandmother "fell ill". This phrase accurately captures the sudden onset of a serious sickness that would necessitate an operation. While "was unwell" is generally correct, "fell ill" is more specific to the context of the story.
Step 3: Final Answer:
The most appropriate phrase to complete the sentence is "fell ill".