Why isn't "konstnärerna" correct in this context? Is it because it is a term related only to fine arts and not music?
Yes, "konstnär" often refers to a "visual artist" like a painter or a sculptor.
Why can't the answer be 'med instrumenten' ? In other sentences it's not necessary to use the sina, but here it's marked as wrong. Is there a reason why it doesn't work here?
Is "MED sina instrument" idiomatic? My first thought would be "på". Though I guess that would mean they are actually sitting ON their instruments? ;o)
Why sina and not deras?