It's my understanding that in the vast majority of uses of possessives (mio, tuo, suo, etc), you always put an article (conjugated to that which is possessed) before the possessive itself, regardless if the noun directly follows it. The only exceptions I know of are family members (claiming someone as "my mother" or "my father," I don't actually know the words for "mother" or "father" yet).
Yes, in general the article is always specified except for singular family members and some other special costruction (such as casa mia). Anyways, in this case "tuoi" is a subject complement and you can also omit the i, since there already is the gli. Putting the i you slightly emphasize the tuoi.
In Italian an article is almost always mandatory before a possessive. THE EXCEPTIONS ARE:
It's not used before close family members, in the singular and not modified, e.g. "mio padre" (my father), unless the possessive is "loro" (in which case the article is needed).
IT'S OPTIONAL (!!! = confusion!!!) WHEN THE POSSESSIVE ADJECTIVE IS ALONE FOLLOWING A FORM OF "ESSERE" (e, sono), e.g. "è mio" (it's mine).
IT'S NOT USED IN A SMALL NUMBER OF SET PHRASES, e.g. "casa mia" (my home).
You will find the forgoing notes just below the possessive-lesson-icons (icons you click on to start any of the 5 or 6 lessons on possessives), plus additional explanation about possessives, in gray script.
Sorry to be so un-concise, but at least I'm ON TOPIC, y'all! :)
"The elephants are yours." = "Gli elefanti sono i tuoi."
"They are your elephants." = "Sono i tuoi elefanti."
There is a difference between the possessive pronoun "yours" and the possessive adjective "your", so Duolingo will want the same matching form in each language. Grammar is being taught as well as definition.