"What did you eat at lunch?"
Translation:¿Qué comiste en el almuerzo?
My wife, a native speaker from Colombia says that "Qué almorzaste?" is acceptable and commonly used in her country. Similarly for breakfast or dinner one could ask "Qué desayunaste?" and "Qué cenaste?"
I tried this and it was rejected although I used cenaste for a similar question regarding dinner and it was accepted. I'm not sure if I want to report it or not because maybe the focus here is on using the verb comer and I can't remember the exact question regarding dinner. At any rate, I think you could ask someone this and they would know what you were asking even it does not happen to be what DuoLingo is looking for here.