What type of code executes on an end user's machine from a web browser?

JavaScript is a programming language that is primarily used for enhancing web pages and enabling interactive content in browsers. When a user visits a website, JavaScript can execute in the user's web browser, allowing for dynamic page updates, form validation, animations, and various user interactions without requiring a page reload. This functionality is essential for creating a smooth and engaging user experience on the web.

Active X, while also capable of running in a web browser, is specifically tied to Internet Explorer and has largely fallen out of favor due to security concerns. Although it can execute code on a user's machine, it is not as widely utilized as JavaScript across all modern browsers.

Malware refers to malicious software designed to harm, exploit, or otherwise compromise devices and networks, and it does not represent a legitimate execution of code like JavaScript does.

Mobile does not specifically define a type of code that executes in a web browser; rather, it refers to applications or websites that are intended to be used on mobile devices, which can include various programming languages and technologies.

Therefore, JavaScript is distinctly recognized for executing on an end user's machine directly from web browsers, making it the correct answer.