Explanation:

XML:
XML stands for Extensible Markup Language. It is a markup language that defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. XML uses tags to define elements and attributes to provide additional information about elements.

Entities in XML:
Entities are used in XML to represent special characters or symbols that have a special meaning in XML. These entities are predefined and can be used in XML documents to represent characters that would otherwise be interpreted as markup. There are five predefined entities in XML:

1. < (less than): It represents the less than symbol "< />
2. > (greater than): It represents the greater than symbol ">".
3. & (ampersand): It represents the ampersand symbol "&".
4. " (double quote): It represents the double quote symbol '"'.
5. ' (apostrophe): It represents the apostrophe symbol "'".

Undefined Entity:
The entity mentioned in option 'D' is © (copyright). This entity is not predefined in XML and is therefore considered an undefined entity.

Usage of Undefined Entity:
If an undefined entity is used in an XML document, it will result in a parsing error. To use the copyright symbol in an XML document, you can either use the corresponding numeric character reference "©" or define the entity yourself using a Document Type Definition (DTD) or an XML Schema.

Conclusion:
In XML, the entity © is not predefined and is therefore considered an undefined entity. It cannot be used directly in an XML document without defining it first.