Interview - Writing sample - what should I write?

Yes, definitely professional in nature. Whatever you do, just make sure your writing is top notch with zero errors. That is the whole point. If you can also make it meaningful and interesting for the reader you have a chance to stand out.

When I was applying to grad school I had a generic personal statement about my background, interest in RE, and both what I wanted from and how I wanted to contribute to the program. I've used that (with some minor tweaks for each unique scenario) for a bunch of writing samples on job interviews and have always received very positive feedback because it is well-written and tells my full story in RE that otherwise might not be completely conveyed in the interview process.