This question is an easy one.  The interviewer simply wants to know if the challenges you seek coincide with the challenges and opportunities the job presents.  Your knowledge of the position and organization should allow you to present an answer which matches the job.  Detail some examples of some challenges you would like to face and some contributions you will make.  Share with the interviewer why the position your interviewing for matches up well with your strengths along with areas where the job will help you grow and develop.  Make sure you have a solid understanding of the position so as to give a complete and well thought out answer.