Question:What does static variable mean? 

There are 3 main uses for the static.

1. If you declare within a function: It retains the value between function calls

2. If it is declared for a function name: By default function is extern..so it will be visible from other files if the function declaration is as static..it is invisible for the outer files 

3. Static for global variables: By default we can use the global variables from outside files If it is static global..that variable is limited to with in the file.
#include <stdio.h>
int t = 10;  
main(){
int x = 0; 
void funct1();
funct1();            
printf("After first call \n");
funct1();            
printf("After second call \n");
funct1();            
printf("After third call \n");
}
void funct1()
{
    static int y = 0;  
    int z = 10;             
    printf("value of y %d z %d",y,z);
    y=y+10;
}
value of y 0 z 10 After first call
value of y 10 z 10 After second call
value of y 20 z 10 After third call

Question:What are the different storage classes in C? 

C has three types of storage: automatic, static and allocated.  Variable having block scope and without static specifier have automatic storage duration. 

Variables with block scope, and with static specifier have static scope. Global variables (i.e, file scope) with or without the the static specifier also have static scope.  Memory obtained from calls to malloc(), alloc() or realloc() belongs to allocated storage class.

Question:Can static variables be declared in a header file? 

You can’t declare a static variable without defining it as well (this is because the storage class modifiers static and extern are mutually exclusive). A static variable can be defined in a header file, but this would cause each source file that included the header file to have its own private copy of the variable, which is probably not what was intended.

Question:Can a variable be both constant and volatile? 

Yes. The const modifier means that this code cannot change the value of the variable, but that does not mean that the value cannot be changed by means outside this code. 

The function itself did not change the value of the timer, so it was declared const. However, the value was changed by hardware on the computer, so it was declared volatile. If a variable is both const and volatile, the two modifiers can appear in either order.

Question:Where does global, static, local, register variables and C Program instructions get stored? 

Global , static, local :  In main memory          
Register variable: In registers
C program : In main memory.

Question:What is use of void data type? 

Void is an empty data type normally used as a return type in C/C++, C#, Java functions/methods to declare that no value will be return by the function.
The another used of void is to declare the pointer in C/C++ where It is not sure what data type is addressed by the pointer.

Question:four type of scope in c: 

Block scope.
Function scope.
File scope.
Program scope.

Question:What are variables and it what way is it different from constants? 

Variables and constants may at first look similar in a sense that both are identifiers made up of one character or more characters (letters, numbers and a few allowable symbols). Both will also hold a particular value.  Values held by a variable can be altered throughout the program, and can be used in most operations and computations. Constants are given values at one time only, placed at the beginning of a program. This value is not altered in the program. For example, you can assigned a constant named PI and give it a value 3.1415  .  You can then use it as PI in the program, instead of having to write 3.1415 each time you need it.

Question:Can you use  “int” data type to store the value 32768? Why? 

No. “int” data type is capable of storing values from -32768 to 32767. To store 32768, you can use “long int” instead. You can also use “unsigned int”, assuming you don’t intend to store negative values.

Question:What are global variables and how do you declare them? 

Global variables are variables that can be accessed and manipulated anywhere in the program. To make a variable global, place the variable declaration on the upper portion of the program, just after the preprocessor directives section.