STRING * res[6]; //Ok
res[0] = "640 x 480"; //No
res[1] = "800 x 600"; //No
res[2] = "1024 x 768";//No

You need to make a function that inits the strings:

res[0] = str_create("640 x 480");
res[1] = str_create(..);
..
Then, the digits will work