The prompt says "glass", so which is it - a glass or a cup? If the former, does it mean both the drinking glass as well as the material?
Is the distinction between "cup" and "glass" in English clear cut? In Hebrew there is also ספל. The distinction between ספל and כוס is not clear cut. If it's >200 ml, made of glass, and typically holds or is now holding cold beverage - or you're talking about the corresponding amount of the cold beverage itself - it will definitely be כוס (and in English "glass", right?). If it's <100 ml, made of china, and holds or is holding a hot beverage - or you're talking about the corresponding amount of hot beverage - it will definitely be ספל (and in English "cup", right?). Between these two extremes, it's vague in Hebrew. But it follows that כוס has a stronger affinity to "glass" than to "cup".
(However, when specifying quantities in recipes, English uses only "cup", and Hebrew always "כוס".)
I disagree with your definitions. כוס is the general term (which includes cup/glass/mug and anything similar); ספל is a type of כוס with a handle for hot drinks. ספל is definitely not smaller than כוס.
If you look up the Talisman טאליסמן / ברברים, song Barbarians, it's used as both. (As a glass of beer... Bartender another glass.. just another cup of beer) the video is in a tavern...
Always "kos". "kus" is a vulgar word referring to the female sex organ.
You can always look up dirty words in English to Hebrew with Reverso... ha.
a glass, a cup is pronounced kos. with pronunciation kus, it would have a different meaning, which is not in Duolingo. http://context.reverso.net/%D7%AA%D7%A8%D7%92%D7%95%D7%9D/%D7%A2%D7%91%D7%A8%D7%99%D7%AA-%D7%90%D7%A0%D7%92%D7%9C%D7%99%D7%AA/%D7%9B%D7%95%D7%A1#pussy