"Yo siempre he comido carne de res."
Translation:I have always eaten beef.
I think "vaca" is a female cow, "toro" is a male cow (bull) and "res" is the general name of the species of animal (cow - regardless of gender). In the same as in English you have hens and roosters which are chickens, in Spanish you have "gallina" (hen), "gallo" (rooster) and the general species name "pollo" (chicken).
I'm not 100% sure on this, but it is my understanding of the matter, so I think "carne de vaca" would be beef that is specifically from a female cow, although I thought it was what it was called when I was in Spain!
In short, I think that "carne de vaca" should be accepted, but this was just meant to explain why I think "cow" is sometimes "res" and sometimes "vaca".
Here in Mexico, the standard phrase on restaurant menus and street stall signs is "carne de res."