Is the handle the only difference between cup (Tasse) and glass (Glas)? Can't this sentence be translated as "One cup"? (English and German are not my mother tongues).
Edit: http://english.stackexchange.com/questions/75729/whats-the-difference-between-cup-and-glass answered my question.
As the link only covers the English usage, here a bit on the German part: 'Tasse' is used both for mugs and coffee and tea cups. 'Glas' in German usually means that the thing is indeed made of glass. If it's made of plastic or some other material, we tend to use 'Becher' instead. For cups not made of china you can also use 'Becher' (e.g. Kaffeebecher). For plastic cups you can also say 'Plastiktasse'. Paper cups are called 'Pappbecher' in German. Note: these are rules of thumb. There's always some overlap.
In US English, a glass is usually made of glass, and usually used for cold drinks rather than hot drinks. A cup, which usually has a handle, is more likely to be used for hot drinks. A jar usually has a lid, and might be used for food storage.
I'm confused because I thought that one translated to "Eins". Does this simply mean that both "Ein" and "Eins" are interchangeable?
Eins is the number when you count or calculate. Eins, zwei, drei,... Or your exam result. ; But it is always ein Apfel, eine Banane, ...
"Jar" can translate to 'Glas', so you're solution isn't wrong. But without further context I'd say that 'glass' = 'Glas' is the more natural solution. BTW: duolingo is a place where people learn new languages. Many of them (like myself) aren't native speakers of English. So, please be considerate of them and use proper spellings.
Sure, you can report it. It won't get included automatically, though. There are still some humans in the line who check the suggestions.