Option A is correct: when used with XSD schemas, XML enforces strict data type checking on elements and attributes, which is one of XML's key advantages over looser formats. Option B is incorrect because XML was designed specifically to be extensible - developers can create custom tags and structures. Option C is incorrect because XML doesn't replace HTML; they serve different purposes (data vs. presentation). Option D is incorrect because XML doesn't extend HTML - XHTML does, but plain XML and HTML are separate specifications.