"Robi" is accepted, but this exercise is designed to teach the verb "gotować". I disagree, as "obiad" rather usually really takes some "cooking", I don't see a slightest problem here. With "śniadanie" it would look weird though, true.
P.S. The plural of "jajko" is "jajka" :)
Can't English just decide on one spelling? ;) We accept five translations of "mama" ([mom/mother/mum/mama/ma]), but clearly no one was asking for "mam" when we were adding those. I can add it here, but doing it everywhere would take hours of work, so I'd encourage you to go with one of the more common translations instead...