The correct answer is D. Card index.
A card index is a system of organizing information on cards. Each card has a unique identifier, such as a customer number, and the information about the customer is written on the card. Card indexes are often used in businesses to keep track of customers, products, and inventory.
A vowel index is a type of index that uses vowels to organize information. For example, the names of customers might be organized alphabetically by the first vowel in their last name. Vowel indexes are not commonly used because they are not very efficient.
A book index is a type of index that is found in the back of a book. It lists the names of people, places, and things that are mentioned in the book, and the page numbers where they can be found. Book indexes are very useful for finding information in a book.
A strip index is a type of index that is used to organize information on strips of paper. Each strip of paper has a unique identifier, such as a customer number, and the information about the customer is written on the strip. Strip indexes are often used in businesses to keep track of customers, products, and inventory.
Card indexes are the most efficient type of index for storing small amounts of information about customers. They are easy to use and can be updated quickly.