Final answer:
HTML 4.01 was the version that introduced different document types, expanding upon the concept first seen in HTML 2.0.
Step-by-step explanation:
The version of HTML that introduced different document types is HTML 4.01. The concept of DOCTYPE was first introduced with HTML 2.0, but HTML 4.01 expanded upon it by providing more detailed document type definitions that allowed browsers to render the content correctly.
This included the distinction between Strict, Transitional and Frameset DTDs (Document Type Definitions), offering different levels of conformance to HTML standards and support for different types of content.