snake is translate "serpente", I did an exercise like that before. "Cobra" should be accepted like a kind/type of snake, in english or spanish is the same "cobra". Unless there is a rule that cobra should be with capital "C" if we talking about the type of snake ("serpente").
"ofídio" is a portuguese word that is rare. For serpents in general. Probably a scientific name.
I don't know about mbóis and mboias, but we have "jibóia(s)", which is the popular name of the "Boa constrictor" species.
I have never heard "malacatifa" in my life.....even google doesn't talk much about it, it's used to the north of the country.
I find that in Brazilian culture it is often the sentiment that good enough is OK. Having been married to a Brazilian woman for about 5 years now there are many examples of this type of language difference. it was surprising to me to learn that there is chicken beef and pork beef as well as cow beef ( obviously using beef for the term meat)
Do you mean "bife"? That is just a sound-alike with "beef" and the best translation is "steak" (meaning a piece of meat in general rather than specifically beef). Better ways to describe the things you are talking about are: "chicken steak", "pork steak" and "beef steak" (or simply "steak").
Is “cobra” ever used in Portuguese to describe specifically what would be called a “cobra” in English? The impression I get from searching around is that the answer to that is yes. If so it should be valid to translate “cobra” to “cobra”, even if it is true that the Portuguese word “cobra” is usually used to describe snakes in general.
Edit, over a year after my original comment (2015‑09‑10): Although I never got a satisfactory answer to this from comments here, the offsite information that prompted this question has now been altered such that it no longer makes the claim that Portuguese “cobra” can translate to English “cobra”, so it would appear that it would actually be wrong to accept “cobra” as a translation of “cobra” in this course.