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

To determine which of the following is NOT a primitive data type in JavaScript, we first need to identify the primitive data types in JavaScript. The primitive data types in JavaScript are:

1. Number
2. String
3. Boolean
4. Undefined
5. Null
6. Symbol (introduced in ECMAScript 6)
7. BigInt (introduced in ECMAScript 2020)

Now, let's consider the options provided:

1. Object
2. Number
3. String
4. Boolean

From the list of primitive data types, we can see that "Number," "String," and "Boolean" are all primitive data types. However, "Object" is not a primitive data type; it is a complex data type.

Therefore, the correct answer is:

1. Object