My hungarian girlfriend said that the "correct" solution is completely stupid, she said that the correct translation is "Igen, ez egy lámpa"
Duolingo's sentence is a bit unnatural, but it's not wrong. Yours is the correct translation.
"Igen, ez lámpa" makes as much sense as "Yes, this is lamp" does in English... Could it be that someone who goes by the name 'lamp' is answering his/her phone?
Amazing answer, had to chuckle at your reply.