Final answer:
An XML document is considered valid when it is both well-formed and complies with its associated DTD or XML Schema, ensuring data integrity and interoperability.option a.
Step-by-step explanation:
The definitions in an XML document are said to be valid when the tagging system and definitions in the DTD (Document Type Definition) are all in compliance. An XML document is considered well-formed if it follows the correct syntax rules, which requires all tags to be properly opened and closed, nested correctly, and case-sensitive. However, validity goes a step further, requiring the document to be well-formed and to adhere to a specific DTD or XML Schema that defines the structure and the type of data allowed.
A valid XML document ensures that the contained data is both structured and presented according to the predefined rules and is thus able to be processed by other systems or applications reliably. Ensuring that an XML document is valid is crucial for interoperability and data integrity in systems that exchange XML-based data.