What is this question asking for?-->Test this code, find an error in the program and "construct an instance to demonstrate the error" (in C)
As noted by Tom van der Zanden in a comment, and given you already know what's wrong with the code, think of an input that will cause the program to fail, just run the program with that input and you have your instance. Does it make more sense if you think of the phrase, "for instance"?

You may be overthinking it (a good sign actually), if this is just a very basic C language course, they are probably being rather obvious in what they are asking for.

KISS -- Keep It Simple, Sailor