In which language, Backbone.js is written?

The backbone.js is written in JavaScript. It is a JavaScript library that contains a RESTful JSON interface.

Backbone.js is primarily written in JavaScript, as it is a JavaScript library designed to provide the structure for web applications by providing models with key-value binding and custom events, collections with a rich API of enumerable functions, views with declarative event handling, and connects it all to your existing API over a RESTful JSON interface. So, the correct answer would be JavaScript.