The tool in MS Word that allows users to send letters or documents to many contacts is called:

This question identifies a key feature in Microsoft Word for mass communication.

• Correct Option A: "Mail Merge" is the correct tool. Mail Merge allows users to create a batch of personalized documents, such as letters, emails, or labels, by combining a main document with a data source (like a list of names and addresses). This automates the process of sending the same content to multiple recipients with individualized details.
• Option B: "Email" is a method of communication, not a specific tool within MS Word for generating multiple personalized documents from a template.
• Option C: "Data Source" is a component of Mail Merge (the list of recipients), but not the tool itself.
• Option D: "Address Book" is a collection of contact information, which can serve as a data source, but it is not the Mail Merge tool.