This is a tough question. But the trick is that your interviewer does not know anything about you till you tell them. It is better to be conservative in answering this question.
Do not boast about your application in other companies and your bright chances of getting selected there. It is simple to say that you are only applying at this company.
Sample answer is:
After my careful analysis I found your company to be a good fit for my career goals. So I have applied only to your company. Also I want to focus on one thing at a time to do justice to my job application.