The correct answer is 3. To discover whether the source is going to cover the topics you need to research
Explanation.
The process of conducting research which implies looking for information to answer to a question or to prove a thesis include multiple stages one of this is looking for materials such as books, articles, academic journals related to the main topic or focus of your research. For doing it is recommended to include only reliable sources and also looking at the abstract in case of materials such as articles or journals or at the index in case of longer documents such as books before including the material or begin to read it, as this would allow the research to know if the material focuses n the same topic he does and can be used or otherwise does not cover the topics and need to be discarded. Considering this it can be concluded the purpose of looking at a book index is "to discover whether the source is going to cover the topics you need to research".