What is a valid user-defined function name?

A valid user-defined function name must be any valid identifier. An identifier in programming is a name that is used to identify a variable, function, class, module, or other object. Valid identifiers typically start with a letter (a-z, A-Z) or underscore (_) and can be followed by letters, digits (0-9), or more underscores. They cannot include spaces or special characters, and they cannot start with a number.

Using a reserved word would lead to errors, as reserved words have predefined meanings in programming languages and cannot be used as names for functions or variables. Identifiers must also adhere to specific naming rules which disqualify them from being merely any variable name or a floating-point number, both of which have distinct functions and contexts in programming. Therefore, the correct answer highlights the importance of adhering to the identifier naming conventions when creating user-defined functions.