Which of the following is NOT a built-in data type in Python?

The correct answer identifies that "List of functions" is not a built-in data type in Python. Python has a variety of built-in data types that include integers, floats, strings, and lists, which are fundamental to Python programming.

An integer is a whole number, either positive or negative, while a float represents numbers with decimal points. A string is a sequence of characters used to represent text. Lists in Python are used to store collections of items, allowing for ordered and changeable sequences. Each of these data types plays a crucial role in data manipulation and storage in Python.

On the other hand, "List of functions" does not refer to a standard or built-in data type within the language. While functions themselves can be elements in a list (meaning you can create a list that includes functions as its elements), there is no distinct data type recognized in Python that specifically denotes a "list of functions." Thus, this choice stands out as being outside the realm of Python's built-in data types.