The answer is A: to help locate alphabetical listings.
A guide word is a term that helps you locate a specific term or word you may be looking for in a reference work (such as a dictionary, although, do notice that dictionaries, like almost all reference works actually use two guide words that are to be found at the top of each page, the first guide word is the first term to be found in the page and the second guide word is the last to be found in the page).
