Final answer:
HTML and CSS are used for Front-end development, contributing to the User Interface (UI) of a website, which is part of what the user directly interacts with.
Step-by-step explanation:
Using HTML and CSS to provide visual elements to the end user would be considered part of a website's User Interface (UI) and specifically the Front-end development. HTML (HyperText Markup Language) and CSS (Cascading Style Sheets) are core technologies for building web pages. They are responsible for the structure and style of the website, which directly interact with the user's experience. The Back-end, in contrast, refers to server-side development which focuses on databases, server logic, and application integration, while Middleware is a software layer that lies between the operating system and applications on each side of a distributed computer network. Therefore, the correct option is B) User Interface (UI).