Answer:
Digital reference
Step-by-step explanation:
An abstract index is a form of service that provide shortened or summarized contents of documents and assign of descriptors for referencing documents.
A virtual library is an online database of text, images, audio, video, or and other digital contents.
A periodical database finds use when you want to find articles on personal topics of interest in magazines, journals, or newspapers.
Digital reference or virtual reference is a service by which a library reference service is conducted online.