Which statement is true?
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XML elements must have a closing tag
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XML tags must be lower case
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XML elements must be properly nested
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XML attribute values must be quoted
XML requires all elements to have closing tags (A) and be properly nested (C). Attribute values must be quoted (D). However, XML tags are case-sensitive but not required to be lowercase (B is false) - 'Name' and 'name' are different tags, but uppercase is valid. These rules ensure well-formed XML documents.
Well-formed XML requires every element to have a matching closing tag, elements must be properly nested (no overlapping tags), and attribute values must always be enclosed in quotes. XML tags are case-sensitive but there's no rule requiring them to be lowercase — and are just treated as different tag names, which is why that option is false.