prog.c: In function ‘main’:
prog.c:4:10: error: ‘This’ undeclared (first use in this function)
    4 |   printf(This sentence will produce an error.);
      |          ^~~~
prog.c:4:10: note: each undeclared identifier is reported only once for each function it appears in
prog.c:4:14: error: expected ‘)’ before ‘sentence’
    4 |   printf(This sentence will produce an error.);
      |              ^~~~~~~~~
      |              )