Mocking birds are innocent birds who do nothing but sing and make people happy. Killing one is a sin because there was no reason to do so.
This question deals with synonyms (awful and terrible means similar things) so consider what a similar-meaning word would be for fabulous. I don't know if this is a multiple choice question or if you have to fill in a blank, but think of words like amazing or great - they mean similar things to fabulous. Keep this in mind as you answer this question.