return calloc(1, 1); is just a stub so that the initial solution compiles and behaves normally. it also illustrates what kind of string you need to return in your solution: it has to be nul-terminated and heap-allocated (i.e. via malloc() &co).

calloc(1, 1) is a construct that creates a heap-allocated empty string

to pass the kata you will of course have to return another string

Do you understand what you need to return in C version of the kata?
Do you know and understand what calloc does?