Which of the following is NOT a primitive data type in JavaScript?

In JavaScript, primitive data types are the most basic kinds of data that are not objects and have immutable characteristics. These include Boolean, String, Number, and others like Undefined and Null.

The correct answer is that "Object" is not a primitive data type. Instead, objects in JavaScript are complex data structures that can hold collections of data and more complex entities. Unlike primitive types, which represent single values (e.g., true/false for Boolean, a sequence of characters for String, or a numeric value for Number), objects can have properties and methods, and can be modified after their creation.

For instance, you can create an object that may hold multiple values across various properties, such as:

let person = {
    name: "John",
    age: 30
};

This allows for much more complex structures compared to primitive types. Therefore, while Boolean, String, and Number serve as fundamental data types representing single, immutable values, Object serves as a reference type that can contain multiple types of data and encapsulate functionality, making it distinctly different from primitive data types.