Which keyword is used for storage class?

In C programming, storage class specifiers define the scope and lifetime of variables. The keyword 'auto' is a valid storage class specifier used for automatic local variables. In contrast, 'scanf' and 'printf' are standard library functions, not storage classes. Additionally, 'external' is not a valid keyword; the correct term for external linkage is 'extern'. Therefore, 'auto' is the correct choice.