What kind of questions can be expected in interview round (SA) of KVPY?? Which is the best topic to opt for??

The questions are very general and they will difficult questions on your strong topics and very very easy on your weak ones.

I will share my experience hope it will help
They asked me what should they ask, I said physics and maths but no chemistry
They asked me few math problems on geometry and calculus for which you can use the board
They asked me reasoning type questions on physics some of which I didnt know but thats fine
Then they just asked me few very basic organic chemistry questions which I was able to solve easily.

Then there are general question regarding your goals and ambitions etc.

Hope this helps