> Write a C program which prints Hello World! without using a semicolon

Here's one way, cheating a bit by abusing the definition of "a C program which prints":

    /tmp$ gcc hw.c
    hw.c:1:2: warning: #warning Hello world! [-Wcpp]
     #warning Hello world!

A more serious solution, though:

    /tmp$ cat hw.c
    #include <stdio.h>
    void main(void)
    {
        if (puts("Hello world!")) {}
    }
    /tmp$ gcc hw.c
    /tmp$ ./a.out
    Hello world!