To answer this question, you need to understand what DTD stands for.
The correct answer is A) Document Type Definition.
A Document Type Definition (DTD) is a set of markup declarations that define the structure and data types for a particular type of XML document. It specifies the elements, attributes, and entities allowed within the XML document, as well as the relationships and constraints between them. DTDs are used to ensure that XML documents conform to a specific structure and can be validated against the defined rules.