"A piece of string"
Translation:En bit snöre
But it is confusing. 'Ett snöre' I rather translate as 'one (whole piece of) string'. But 'a piece of string' is, I think, when I cut e.g. an inch out of it. On the other hand, 'one piece of string' again refers to the whole string. Maybe I'm just overthink the sentence. English is not my native.
This is probably nothing to do with the Swedish course specifially, more about how Duo works in general. They run a lot of A/B tests (they let different groups of users have different experiences, and then they check which group got the best results) so that how the lessons work can change considerably. I'm currently getting sentences over and over again until I get them right at least once, but it may work differently for you if you're in a different test group. There are usually many discussions about this in the general forums.
This question is not just. A piece of string translates as en bit snöre. At least for me. While ett snöre means a string. However they can mean the same, the last one is not the real "word by word" translation. So why am I wrong if I choose only that one what means really what the question is asking? Sometimes the questions are really not pedagogical and helpful but really confusing.