You can do it even faster with the if statements:

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

    int main(int argc, char *argv[])
    {
        if (argc < 2) {
            fprintf(stderr, "Usage: %s <string>\n", argv[0]);
            return 1;
        }

        char *s = argv[1];
        int i;

        /* find the end of the string */
        for (i = 0; s[i] != '\0'; ++i)
            ;

        /* make sure the string wasn't empty */
        if (i == 0) {
            fprintf(stderr, "Error: empty string\n");
            return 1;
        }

        /* last character is at s[i - 1] */
        char d = s[i - 1];

        if (d == '0')
            printf("even\n");
        if (d == '1')
            printf("odd\n");
        if (d == '2')
            printf("even\n");
        if (d == '3')
            printf("odd\n");
        if (d == '4')
            printf("even\n");
        if (d == '5')
            printf("odd\n");
        if (d == '6')
            printf("even\n");
        if (d == '7')
            printf("odd\n");
        if (d == '8')
            printf("even\n");
        if (d == '9')
            printf("odd\n");
    
        return 0;
    }

gcc -std=c11 -Wall -Wextra -O2 -o check_digit check_digit.c

./check_digit 999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999