Why this code has errors

Question

#include <stdio.h>
#include<string.h>
int main() {
char a[60];
char b[60];
char c[]="is the friend of";
char d[];
printf ("The first friend");
gets(a);
printf ("The second friend");
gets(b);
strcpy(d,(strcat(a,(strcat(b,c)))));
puts(d);
return 0;
}