Which of the following is a commonly used JavaScript library?

jQuery is a widely used JavaScript library that simplifies HTML document traversing, event handling, animating, and Ajax interactions for rapid web development. By providing an easier syntax to work with, jQuery allows developers to write less code to accomplish tasks that would otherwise require a lot of JavaScript. This enhancement in productivity is one of the key reasons why jQuery has maintained its popularity among developers for years.

In contrast, while Bootstrap is a framework that assists with building responsive websites using HTML, CSS, and JavaScript, it is not classified as a JavaScript library. CSS3 is a style sheet language used for describing the presentation of a document written in HTML or XML, and it doesn't pertain to JavaScript functionalities. HTML5 is the latest version of the HTML standard, focusing primarily on structuring web content and not on any JavaScript operations directly. Therefore, jQuery distinctly stands out as the JavaScript library in this context.