Explanation:
In C, storage class specifiers define the scope (visibility) and lifetime of variables or functions. There are four storage class specifiers in C: auto, register, static, and extern.

a) auto:
The auto storage class specifier is used to define local variables within a block. It is the default storage class specifier for all local variables if no other storage class specifier is specified. The variables declared with the auto storage class specifier are created and destroyed automatically when the block containing them is entered and exited, respectively.

b) register:
The register storage class specifier is used to define local variables that should be stored in CPU registers for faster access. However, it is merely a request to the compiler, and the compiler may or may not choose to honor it. The register keyword is typically used with frequently accessed variables to optimize performance.

c) static:
The static storage class specifier has different meanings depending on its context:
- When used with local variables inside a function, it retains the value of the variable between function calls. In other words, the variable retains its value even after the function call ends.
- When used with global variables or functions, it restricts their visibility to the file in which they are defined. They cannot be accessed from other files using the extern keyword.

d) volatile:
The volatile keyword is not a storage class specifier. It is a type qualifier that informs the compiler that a variable's value may change unexpectedly and should not be optimized or cached. It is commonly used when dealing with hardware registers or variables shared between different threads or interrupt service routines.

Therefore, the correct answer is option 'D' (volatile) as it is not a storage class specifier in C.