The correct answer is: C. aid to filing
Indexing is an important aid to filing because it helps to organize and retrieve information quickly and easily. By creating an index, you can create a system for storing and retrieving information that is both efficient and effective.
Option A is incorrect because indexing is not a method of filing. Filing is the process of organizing and storing information in a way that makes it easy to find. Indexing is a tool that can be used to help with filing, but it is not the same thing as filing.
Option B is incorrect because indexing is not a method of maintaining account books. Account books are used to record financial transactions. Indexing is a tool that can be used to help with maintaining account books, but it is not the same thing as maintaining account books.
Option D is incorrect because indexing is not a method of maintaining statistical books. Statistical books are used to record and analyze data. Indexing is a tool that can be used to help with maintaining statistical books, but it is not the same thing as maintaining statistical books.