Answer:
Her book was illustrative of 18th century prose.
Step-by-step explanation:
Let's take a look at the meaning of each option and use elimination.
1. illustrious: notably or brilliantly respected and admired because of dignity, achievements, or actions. --> We can eliminate this option. It is very hard to imagine a book being illustrious.
2. illustration: a picture or an example helping make something clear or attractive. --> Even though this option makes more sense than the one above, we need to pay attention. "Illustration" is not an adjective, but a noun. To correctly complete the sentence, we should choose an adjective. In order to use this noun, we would need the article "an", which the sentence does not have.
3. illustrative: serving as an example or explanation; explanatory. --> This is the best option. It makes sense for a book to explain something, or to be an example of a type of literary work. In this case, her book is an example of 18th century prose.