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.
"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.