What is meant by abstraction when creating user$-$defined functions?

a$.$ The creation of user$-$defined functions with more generic names, such that not only do those names hide the details of the existing functionality, but each function name is more human$-$readable.

b$.$ The creation of user$-$defined functions to arrange some complex logic, so that those functions hide the complexity and only expose the information that is needed to make use of the logic.

c$.$ The creation of user$-$defined functions with meaningful names, such that not only do those names describe the existing functionality, but each function name highlights how the functionality has been extended.

d$.$ The creation of user$-$defined functions to accurately describe some complex logic, so that those functions reveal the complexity and expose all the details in manageable units of functionality.