The problem is that "ini X Y" at the beginning of a sentence should literally be translated as "This is X Y", not as "This X is Y".
But taking that level of adhearance to the indonesian structure, the model answer sould probably be "This is the teacup that belongs to father." So in a way they are breaking their own rules...
But I agree with what Arkonide said. There's no clear-cut line between translation in the narrow sense and interpretation.
Sometimes we have to kind of probe around in the mist and make an educated guess to hit on the right side of the line in Duo's opinion. I guess the "ini X Y" matter has been quite firmly established so far, so they won't budge on it. The question on whether "milik" is "his cup" or "the cup that belongs to him" seems more loosly handled.
I am still very uncertain about many aspects of Indonesian but would have thought that 'This teacup is father's ' is correct too. I cannot argue with Rick392366 either. The question is to what extent one translates literally at the expense of another language at times. The lines between literal translations and interpretations are blurred.