where do constant variables stored in C

How they are stored is an implementation detail (depends on the compiler).

For example, in the GCC compiler, on most machines, read-only variables, constants, and jump tables are placed in the text section.

Consider the code:

const int i = 0;
 static const int k = 99;
 
int function(void)
{
     const int j = 37;
     totherfunc(&j);
    totherfunc(&i);
   //totherfunc(&k);
    return(j+3);
 }

Generally, i can be stored in the text segment (it's a read-only variable with a fixed value). If it is not in the text segment, it will be stored beside the global variables. Given that it is initialized to zero, it might be in the 'bss'(block starting symbols) section (where zeroed variables are usually allocated) or in the 'data' section (where initialized variables are usually allocated).

If the compiler is convinced the k is unused (which it could be since it is local to a single file), it might not appear in the object code at all. If the call to totherfunc() that references k was not commented out, then k would have to be allocated an address somewhere - it would likely be in the same segment as i.

The constant (if it is a constant, is it still a variable?) j will most probably appear on the stack of a conventional C implementation. (If you were asking in the comp.std.c news group, someone would mention that the standard doesn't say that automatic variables appear on the stack; fortunately, SO isn't comp.std.c!)

Note that I forced the variables to appear because I passed them by reference - presumably to a function expecting a pointer to a constant integer. If the addresses were never taken, then j and k could be optimized out of the code altogether. To remove i, the compiler would have to know all the source code for the entire program - it is accessible in other translation units (source files), and so cannot as readily be removed. Doubly not if the program indulges in dynamic loading of shared libraries - one of those libraries might rely on that global variable.

(Stylistically - the variables i and j should have longer, more meaningful names; this is only an example!)