What term describes rules in an external text file that determine how to display HTML elements?

The term that describes rules in an external text file that determine how to display HTML elements is Cascading Style Sheets (CSS). CSS is a stylesheet language specifically designed for describing the presentation of a document written in HTML or XML. This external file allows web developers to separate content (HTML) from design (styling) by specifying how elements should be rendered, including aspects such as layout, colors, fonts, and spacing.

Using CSS promotes a more organized and maintainable approach to web design, as changes to the visual presentation can be made in the stylesheet without altering the underlying HTML structure. This separation also enhances consistency across multiple pages of a website since one CSS file can control the styles of all linked HTML files.

The other options do not fulfill this role. JavaScript functions are primarily focused on scripting and functionality rather than presentation. HTML declarations refer to the structure and elements of a web page but do not include styling information. Web design templates provide an overall layout and framework for a site, but they are not specifically about the rules contained in a stylesheet. Thus, Cascading Style Sheets is the appropriate and accurate term for the rules in an external text file that determine the display of HTML elements.