Which of the following is not a storage class specifier in C?a.Registerb.Automaticc.volatiled.External

To determine which of the options is not a storage class specifier in C, we need to understand what each option represents.

a. Register: This is a storage class specifier in C. It is used to define local variables that are stored in CPU registers for faster access.

b. Automatic: This is also a storage class specifier in C. It is the default storage class for local variables and is used to define variables that have a local scope and are automatically allocated and deallocated.

c. Volatile: This is another storage class specifier in C. It is used to indicate that a variable may be modified by external sources outside the program's control, such as hardware or other threads.

d. External: This is not a storage class specifier in C. It is used to declare a variable that is defined in another source file and can be accessed by other source files.

Therefore, the correct answer is d. External.