To answer this question, let's go through each option to understand why it is correct or incorrect:
Option A) All XML documents must have a DTD - This option is incorrect because XML documents are not required to have a Document Type Definition (DTD). A DTD is an optional component that defines the structure and rules for the XML document, but it is not mandatory.
Option B) All XML elements must be properly closed - This option is correct. In XML, all elements must be properly closed using either a closing tag or a self-closing tag. For example, or. Failing to close elements properly will result in an XML syntax error.
Option C) All XML elements must be lowercase - This option is incorrect. XML is case-sensitive, which means that element names can be written in either uppercase or lowercase or a combination of both. For example, and are both valid XML element names.
Option D) All XML elements must be uppercase - This option is incorrect. As mentioned earlier, XML is case-sensitive, so element names can be written in either uppercase or lowercase or a combination of both.
The correct answer is B) All XML elements must be properly closed. This option is correct because it is a fundamental rule of XML syntax to properly close all elements.