The correct answer is B. A style manual helps a writer determine the format for a specific citation.
There are several different styles a write might need. MLA is the most common for students. If a writer needs to write a paper using MLA format, an MLA style guide is necessary to format the paper and cite sources properly.
Other styles include APA, AP, and Chicago Manual of Style. Each of these styles has its own guide so that a writer can look up specific aspects of that style.