Let us think about this question in my online application.
Please describe your specific area of academic interest, and explain what motivated you to choose this area.
In my case 100% true answer would be the following,
I am interested in Image Processing.
When I was searching for a supervisor, I met one of the most prominent research professors of the XYZ University Prof. xyz who works in the area of Image Processing, Computer Vision, Graphics and so on. I was interested in Computer Graphics and proposed his a topic. He told me that topic was not fit for MSc thesis to be completed in 4 months. That would actually take more time may be up to a year. Since, I didn't have any other topic in my mind, I requested him to propose a topic for me. He then proposed a topic related to Image Processing. I found that topic very interesting and saw a huge opportunity to learn about the area of Image Processing from the inside.
Is that a good answer?