A list of names, titles, and/or subjects compiled to facilitate reference is known as an index. An index is a <span>list of names, places, and subjects listed in alphabetic order that are treated in a printed work. The index gives information about the page or pages in which each item is mentioned. The index can be also defined as a list of keywords contained in the text of a book or other lengthy writing project. </span>
I believe it is number 3 as it states that fire has been useful in many ways. The others are saying fire has been useful for specific things. The main idea should cover it all.
