Which of the following is used to declare a function in Python?

In Python, the 'def' keyword is used to declare a function. When you start a function definition, you write 'def' followed by the function name and parentheses that can contain parameters. This establishes the function's behavior and how it can be utilized in the program. For example:

def my_function(parameter):

function body

pass

This structure allows the Python interpreter to recognize that a function is being defined and enables developers to encapsulate code for reusability and modular design.

The other options do not correspond to any syntax used for declaring functions in Python, making 'def' the only correct choice.