What is essential to use when working with Sass in Mendix?

Using a compilation tool is essential when working with Sass in Mendix because Sass (Syntactically Awesome Style Sheets) is a preprocessor scripting language that extends CSS with variables, nested rules, mixins, and more. Since browsers do not understand Sass directly, it needs to be compiled into standard CSS before it can be used in a Mendix application.

A compilation tool can automate this process, allowing developers to write their styles in Sass and seamlessly convert them into CSS, ensuring that the application can effectively utilize the stylesheet. This enhances productivity and maintains the organization of styles in larger projects.

In the context of the other options, while JavaScript libraries can be part of a Mendix project for functionality, they do not relate directly to the use of Sass for styling. A graphical user interface may assist in interaction design but does not pertain to the required tools for compiling Sass. Predefined templates could provide a starting point for design but do not fulfill the technical requirement of converting Sass into a usable format for web applications.