Which of the following is not a programming language?

HTML is not a programming language; it is a markup language. Markup languages are designed to define the structure and presentation of text, often in a web context, rather than to implement algorithms or manipulate data through instructions. HTML (Hypertext Markup Language) is primarily used for creating and structuring content on the web, allowing developers to define elements such as headings, paragraphs, links, and images.

In contrast, Python, Java, and Assembly are all programming languages. They provide syntax and semantics for writing code that can perform computations, control hardware, and create complex applications. Programming languages are built around the concept of algorithms and allow for logic and processing of data, which distinguishes them from markup languages like HTML.