Naming conventions for variables in SAS require what?

In SAS, variable naming conventions stipulate that variable names must start with either a letter (A-Z, a-z) or an underscore (_). This rule is essential because it ensures that the variable name is recognized correctly and does not conflict with reserved keywords or data types in the SAS language. By requiring the first character to be a letter or an underscore, SAS prevents ambiguity in variable names and helps maintain consistency across the programming environment.

The other options presented do not align with the established naming conventions in SAS. Variable names do not have to be exactly eight characters long; in fact, they can be up to 32 characters. Additionally, variable names can contain numbers, but they cannot consist solely of numbers or begin with them. Finally, underscores are permissible and often used within variable names to enhance readability. Therefore, the requirement that names must start with a letter or underscore is a key guideline in ensuring valid variable naming in SAS programming.