Final answer:
Babel is the tool that compiles JSX into createElement calls, making it understandable by browsers for use with React components.
Step-by-step explanation:
The tool used to transform JSX into createElement calls is called Babel. JSX is a syntax extension for JavaScript, often used with the React library to describe what the UI should look like. Babel is a JavaScript compiler that converts code written in one version of JavaScript, or with extensions like JSX, into a version of JavaScript that is compatible with current browsers. When you write JSX in a React application, Babel compiles it down into createElement calls, which is how React can understand and interpret the components for rendering.