How to release the memory that has been used for a variable in C?

for (int i=0;i<10;i++) {
 int k=i;
 //do stuff
}

int* intArray;
malloc(200 * sizeof(int));

char array[100];

int reallylongfunction() {
    // Do a lot of stuff
    {
        char stringbuffer[100];
        // Do stuff with the buffer...
        // Ok, we're done with the buffer, and don't want it anymore
    }
    // Do a lot more stuff
    return;
}