You can try something like the following:

NOTE: Obviously, the following lacks in a lot of things like memory de-allocation, error checking, incomplete code, etc. The idea is to give you a hint!

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>

#define ROW 4
#define COL 8
#define CHAR_INDEX_MAX 26

enum alph_enum {A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z};

typedef struct _alphabets {
    char *line[ROW];
}alphabet;

alphabet chars[26];

init_a(enum alph_enum letter)
{
    int i;
    for (i=0 ; i<ROW ; i++) {
        chars[letter].line[i] = (char *) malloc(COL);
        if (chars[letter].line[i] == NULL) {
            printf("memory allocation failed \n");
            return;
        }
    }

    switch (letter) {
                                     /*0123 4567*/
    case H:
        strcpy(chars[letter].line[0], "|       |");
        strcpy(chars[letter].line[1], "|_______|");
        strcpy(chars[letter].line[2], "|       |");
        strcpy(chars[letter].line[3], "|       |");
        break;
    case E:
        strcpy(chars[letter].line[0], "|-------");
        strcpy(chars[letter].line[1], "|_______");
        strcpy(chars[letter].line[2], "|       ");
        strcpy(chars[letter].line[3], "|_______");
        break;
    case L:
        strcpy(chars[letter].line[0], "|       ");
        strcpy(chars[letter].line[1], "|       ");
        strcpy(chars[letter].line[2], "|       ");
        strcpy(chars[letter].line[3], "|_______");
        break;
    /*  for all the other alphabets */
    }

    return;

}

print_str(char word[])
{
    int i, j;

    printf("\n");
    for (i=0; i<ROW; i++) {
        for (j=0 ; j<strlen(word) ; j++) {
            printf("%s", chars[word[j] % 'A'].line[i]);
        }
        printf("\n");
    }
    printf("\n");

    return;
}


int main(void)
{
    init_a(H);
    init_a(E);
    init_a(L);
    /* print_str("HELLO"); */
    print_str("HELL");
    return 0;

    /* free the memory for HEL here */
}

The output will be as follows:

> gcc test.c -o print
> print

|       ||-------|       |
|_______||_______|       |
|       ||       |       |
|       ||_______|_______|_______

>

TODO:

• You can include numbers, special characters (like !, @, #, $, etc), blank space, may be all ASCII chars makes sense.
• You can support bold, italics, underline and highlight for the characters
• You can support foreground and background colors
• You can support animation

Hope this helps!

1. Try to format you code properly. Things like these are hard to read:

   struct font{
   unsigned int c;
   unsigned int *r;
   unsigned int d_n;
   int (*allowed)(int);
   int (*index)(int);
   int (*init)(void);
   int (*exit)(void);
   int (*print)(const char*);
   char ***d;
   };
   

   Hard to read code is more difficult to maintain and you're more likely to miss a bug.

2. Use braces consistently. Things like these just lead to trouble in the long run and don't make the code any easier to read.

   for(int i=0;i<whimsy.d_n;i++)
       for(int j=0;j<whimsy.c;j++)
           free(whimsy.d[i][j]);
   

   Adding additional braces and formatting makes it very explicit what the scope of each loop is and adding additional statements you're less likely to accidentally forget to add a brace.

   for (int i = 0; i < whimsy.d_n; i++) {
       for(int j = 0; j < whimsy.c; j++) {
           free(whimsy.d[i][j]);
       }
    }
   

3. The use of defines for font_alloc and d_malloc are a pretty bad abuse of the pre-processor. You just use it as an automated copy-n-paste mechanism and copy-n-paste code is bad. #define has it's uses but in this case it's the wrong choice.