XML stands for 'eXtensible Markup Language' - a markup language designed to store and transport data in a structured, human-readable format. The 'X' comes from 'eXtensible' (with a lowercase E), emphasizing that users can define their own custom tags. Option B has the correct spelling. Option A uses 'X-TENSIBLE' incorrectly, and D adds a hyphen to 'Markup' and uses 'Languages' (plural).