Which of the following describes a valid identifier?

A valid identifier in programming refers to a name used to identify a variable, function, class, or other entities in code. The characteristic that defines a valid identifier is that it can include letters (both uppercase and lowercase), digits (0-9), or underscores. This flexibility allows programmers to create meaningful names that convey the purpose of the variable or function while adhering to the syntactical rules of the programming language.

Identifiers must not start with a digit, and they cannot include spaces or special characters, which further reinforces why choice C is the correct answer. Reserved keywords, which are defined by the syntax of the programming language for specific purposes, cannot be used as identifiers because they have predefined meanings in the language, making them invalid as identifiers. Thus, understanding the rules surrounding identifiers is crucial in coding to avoid errors and ensure that the code functions as intended.