Which of the following is a correct example of a function definition?

The correct example of a function definition is provided with the keyword "def," which is essential in Python for defining a function. The structure "def my_function():" correctly uses the "def" keyword followed by the function name, and includes parentheses to define any parameters, which, in this case, are absent. The colon at the end signifies the start of the function's body, where you would typically write the code that the function executes.

The other choices fail due to syntax errors: the first option lacks the "def" keyword; the second uses square brackets instead of parentheses, which is not valid for function definitions; and the fourth provides just the function call without defining it first. Thus, understanding the proper syntax for defining functions in Python is crucial for writing functional code.