"Outil" translates to tool so I wouldn't say they're completely interchangeable.
It depends on the context. The first meaning of "instrument" is "music instrument". But it can mean more genrally a "tool" in a precise context.
I asked because I was looking through a dictionary and they listed instrument de musique separately for a musical instrument.
The French Wikipedia page was also titled as such—in its fully glory.
I think I wasn't very clear enough, my bad. The exact translation of "music instrument" is "instrument de musique". Sticto Sensu, "instrument" means "instrument/tool". But in the everyday language, the first meaning of "instrument" is "music instrument".
If you say "Je joue d'un instrument" no one will understand that you play with a screwdriver. Even if you say "J'ai acheté un nouvel instrument", people will understand "music instrument". If you bought a new screwdriver, you'll have to be more precise.
However if you say "En tant que mécanicien, j'utilise beaucoup d'instruments" no one will understand that you repair cars with saxophones. You see, as many other things, it depends heavily on the context.
In this Duo exercice, as we have zero context, it's safer to translate as "the instrument".
Hope it helps.
That makes complete sense. It's that way in most languages (well those I know). I wasn't sure if it would sound a bit weird to only use "instrument" in French for the musical meaning though. Anyway, I do now. Thanks, well appreciated. :)