You can't clear the screen in Sololearn.
For *nix systems: printf("\033[2J");
For Windows: I can't think of a time that'd be useful since you're generally not doing command-line programs. If you must use system("cls") or for(i=0; i < 100; ++i) printf("\n");
Note: C doesn't understand the concept of a screen, all it knows is data streams (FILEs, stdin, stdout and stderr), which is why there's no standard way of doing it.