How to undo a stack when the struct contains a char ** variable?

Question

This code below is something like a candy crush game, where you pop similar letters in a cluster, then they'll disappear and you score. On main, I tried to play it one step ahead and then undo it, and it works since the score managed to undo (from 0 to some score then back to 0 again). However, I just can't figure out why the game board isn't updated?

    typedef struct {
        int rs;
        int cs;
        char **board;
        int score;
    } Instance;
    
with random letters (a, b ,c or d)
    extern Game *create(int nrows, int ncols) {
        if (nrows > MAX_ROWS || ncols > MAX_COLS) {
            return NULL;
        }
        Game *b;
        b->top = 0;
        b->stack[b->top].rs = nrows;
        b->stack[b->top].cs = ncols;
        b->stack[b->top].board = malloc(sizeof(char *) * nrows);
        for (int row = 0; row < nrows; row++) {
            b->stack[b->top].board[row] = malloc(sizeof(char) * ncols);
        }
        srand(time(0));
        for (int row = 0; row < nrows; row++) {
            for (int column = 0; column < ncols; column++) {
                int random = rand() % 4;
                if (random == 0) {
                    b->stack[b->top].board[row][column] = A;
                } else if (random == 1) {
                    b->stack[b->top].board[row][column] = B;
                } else if (random == 2) {
                    b->stack[b->top].board[row][column] = C;
                } else {
                    b->stack[b->top].board[row][column] = D;
                }
            }
        }
        return b;
    }
    
    // Display the current matrix
    extern void display(Game *b) {
        /** Prints top border **/
        printf("   +-");
        for (int top = 0; top < b->stack[b->top].cs; top++) {
            printf("--");
        }
        printf("+\n");
        /** Prints the board **/
        for (int row = 0; row < b->stack[b->top].rs; row++) {
            if (row < 10) {
                printf("0%d | ", row);
            } else {
                printf("%d | ", row);
            }
            for (int column = 0; column < b->stack[b->top].cs; column++) {
                printf("%c ", b->stack[b->top].board[row][column]);
            }
            printf("|\n");
        }
        /** Prints bottom border **/
        printf("   +-");
        for (int bot = 0; bot < b->stack[b->top].cs; bot++) {
            printf("--");
        }
        printf("+\n");
        /** Prints vertical column indices **/
        printf("     ");
        for (int tens = 0; tens < b->stack[b->top].cs; tens++) {
            printf("%d ", tens/10);
        }
        printf("\n");
        printf("     ");
        int count = 0;
        for (int ones = 0; ones < b->stack[b->top].cs; ones++) {
            if (count > 9) {
                count = 0;
            }
            printf("%d ", count);
            count++;
        }
    }
    
    extern int select(Game *b, int r, int c) {
        char colour = b->stack[b->top].board[r][c];
        int n = recursive_helper(b, r, c);
        if (n == 1) {
            b->stack[b->top].board[r][c] = colour;
            return 0;
        }
        b->stack[b->top].score += n*(n-1);
        return n;
    }
 
    
    int main() {
        Game *b = create(5, 10);
    
        display(b);
        printf("\n");
        printf("%d", bp_score(b));
        printf("\n");
    
        select(b, 2, 2);
        display(b);
        printf("\n");
        printf("%d", bp_score(b));
        printf("\n");
    
        b->top--;
        display(b);
        printf("\n");
        printf("%d", bp_score(b));
        printf("\n");
    }

Answer 1

As pointed in the comments, your create function doesn't instantiate a Game struct, you need b = malloc(sizeof(Game));
Another error is that your mallocs use sizeof(int*) and sizeof(int) instead of sizeof(char*) and sizeof(char).

Other than that, your problem comes from the fact that b->stack[b->top+1] = b->stack[b->top]; copies a structure, but the board is a pointer and points to the same object! You only have one char** pointer and a single board in memory.
Thus, when you do b->top-- while you do access another struct, it still points to the same fully updated board.
You need to create a function Instance CopyInstance(Instance src) that creates a new board in the new struct and then copies each cell from the source board in the new one. That way each Instance will point to a different board!