Answer is C, Valid.
XML- Extensible MarkUp Language
designed to store and transport data. doesn't have predefined tags.self descriptive.
DTD- Data Type Definition
DTD defines structure and the legal elements and attributes of an XML document.
XML with correct syntax is "well formed" and XML validated against DTD is well formed and valid.
Check the following link for more info:
https://www.w3schools.com/xml/xml_dtd.asp