Final answer:
JavaScript is used to easily build custom connectors to any REST API source system.
Step-by-step explanation:
The correct answer is d. JavaScript.
JavaScript is a programming language that can be used to build custom connectors to any REST API source system. It is widely used in web development to create dynamic and interactive features on websites.
For example, if you wanted to integrate a web application with a social media API to fetch user data, you could use JavaScript to write the code that communicates with the API and retrieves the necessary information.