The correct answer is C. Structure variables.
A structure is a user-defined data type that can be used to group together related data items. A structure variable is a variable that is declared using a structure. Structure variables can be used to store data in a more organized way than if the data were stored in separate variables.
Option A is incorrect because variables are not necessarily declared using a structure. Variables can be declared using a variety of data types, including structures.
Option B is incorrect because auto variables are variables that are declared inside a function. Auto variables are created when the function is called and destroyed when the function returns.
Option D is incorrect because constant variables are variables that cannot be changed after they are declared. Constant variables are declared using the const keyword.