I think when it's a thing that was created from work, like a work of art or the works of Shakespeare, it is a verk. A product. If arbete could also be used like this, someone should clarify.
I think the English work (when not referring to a product) is uncountable and so couldn't be used in this exercise like "a work". You could only say it without the article or you change the meaning to a product (something from already accomplished work). There might be some ambiguity where it appears countable, at least in outdated literary quotes. I think in every day life we only use "job" when we want something countable and only "work" when uncountable (not a product).
Actually, it's "ssi" that you mean to ask about not "ss". This is also found in Swedish "passion" and "diskussion".
This is the Swedish sj phoneme which actually has two different standard pronunciations. The back version /sj/ (which you called "hw") and the front version /rs/ (which you called "sh").
So, I think these "ssi" words are pronounced differently regionally and between individuals. Duo is playing the /rs/ version in this exercise but you can hear the /sj/ version here: https://forvo.com/search/Professionellt/sv/
sorry but my brain is a bit like soup lately... i saw somewhere on the previous comments that ett refers to the arbete (what you just said) and professionellt is an adjective. Adjectives are on the next lesson, so i don´t know how they work yet. If there´s an ett word, a t is added at the end?
Is there a way to easily tell whether or not an adjective changes its spelling when used in front of an "ett" noun? Or do adjectives change their spelling more often than not? (I'm thinking, for example, how the adjective "rosa" doesn't change its spelling when in front of an "ett" word)