WHO AM I? (a self-searching philosophical treatise) [also called: nan yar? and Koham?]
multi-media presentation: https://ramana-maharshi.weebly.com/who-am-i.html
original texts (including manuscripts) https://www.sriramanamaharshi.org/downloadbooks/whoami_all_languages/who_am_I_Sanskrit.pdf
The context will probably help you understanding whether "sono" applies to "I" or "they". But as a context-free statement, "Cosa sono?" could logically, in my mind, be understood as "What am I?" or "What are they?". Would Duolingo accept both answers? If not, why not?
"What am I?" is infinitely more common for "cosa sono?", because (1) when fielding insults or being slighted is part of the culture, one is constantly defending one's self with these 2 words in an indignant tone; (2) the curiosity behind asking "what are they?" is less likely and, in truth, would more probably be "what are these?", which would be accompanied by 'questi', (3) cosa sono? = natural/common "What am I?" than "What are they?" (4) more natural for 3rd pp: "che cosa sono questi?" or "che sono questi?".
So, relying on grammar, yes, both are possible.
However, there is more ambiguity when using Lei and lei. Part of conversational Italian is clearing up ambiguities, advice to a beginner? get used to it (remember: un casino is not for gambling!!)
Not sure if this is responding to your question, but I hope it's helpful: In formal English, verb "to be" (am/is/are) uses "nominative (subject) case" (I/we/he/she/they) before & also after; in informal English, uses "accusative (object)" (me/us/him/them) after. Examples: It is I (formal) vs. It's me (informal). I am he (formal) vs. I'm him (informal). It is they (formal) vs. It's them (informal). [If you use the formal in speaking, it's technically correct but sounds a bit snobby or old fashioned.] For questions, using the accusative just sounds wrong: What/who/how am I; what/who/how is he; what/who/how are they--all sound fine. Using me/him/them in these sentences doesn't sound right.
@katerinasiap "Cosa sono?" is correct (what are they?, what am I?), "chi sono?" is correct (who are they?, who am I?), "chi cosa sono?" just like that, it's not possible to say, but "chi e cosa sono?" is correct (who and what are they?, who and what am I?). Obviously it depends on the context. Hope it's helpful.
I'm having an existential crisis right now due to this question. What am I? Are we here? Is life an illusion? Are we real? Is this a dream? How did we evolve from a one celled organism that used to photosynthesize, to being humans that use technology in everyday lives (for example to learn languages)? What is the meaning of life? Is there lore in mc?