Which keyword is used in a function to specify a default value for an argument?

The correct keyword used to define a function in Python is "def." This is fundamental as it signals the beginning of a function definition. Within this function definition, you can specify arguments, and subsequently, you have the option to assign default values to those arguments. For example, if you have a function defined as def my_function(arg1=10):, arg1 has a default value of 10.

In terms of the other options, "default" and "set" are not valid keywords in Python that pertain to argument defaults. "None," on the other hand, is a special constant in Python, not a keyword for defining default arguments. While "None" could be used as a default value for an argument, it is not a keyword for setting defaults. Thus, understanding the role of "def" is crucial for creating functions and enabling the use of default argument values in Python programming.