Using food to prop a book open is not advisable, as it can lead to damaging the book or the food itself, creating a mess or attracting pests. However, if you absolutely had to use a food item, you could consider using a large, sturdy fruit or vegetable with a relatively flat surface, like a butternut squash or a large sweet potato. These items are more stable and less likely to be damaged or cause damage compared to softer or more perishable foods.
That being said, it’s best to use a proper book stand, a heavy object, or even another book to prop your book open to protect both the book and your surroundings.
I asked GPT-4. This was its answer (first try),