Translation:Teacher Wang only wants a cup of coffee.
The answer is phrased incorrectly. "Teacher Wang wants only a cup of coffee" is the correct answer. Wang doesn't want anything more than coffee.
The answer given, "Teacher Wang only wants a cup of coffee" is a different statement. That says that Wang doesn't think, doesn't breath, doesn't see, taste or smell. He only wants a cup of coffee. I doubt that this is the expression being uttered in Chinese.
I disagree. The phrase "Wang only wants a cup of coffee" is generally considered the same as "Wang wants only a cup of coffee" so at least the English phrase here makes sense. Most English speakers wouldn't even consider the absurd situation where wanting coffee is the only thing that Wang is doing (not even breathing, as in your example). We either [sensibly] assume that all of the verbs that people passively do all the time are implicitly excluded from a statement like this, or that indeed the speaker is saying that coffee is the only item in the set of things that Wang wants. To suggest that someone is doing exactly one verb, namely "wanting", you should be explicit; "the only thing Wang is doing is wanting coffee", however even this is likely not to imply that Wang is not breathing, as most English speakers would assume that Wang is still breathing.
"Mr. Wang", "Mrs. Wang" etc, should be accepted (more natural than "Teacher Wang" in English, or at least in spoken English. I don't think I, nor any native speaker I know, has ever called or referred to a teacher as "Teacher ____" (at least in British English).
It's true that this is not a natural expression in English and under the same circumstances "Mr. /Mrs./Miss" would likely be used. The problem with this is that it leaves out a key piece of information present in the original Chinese sentence. "Professor Wang" is acceptable in English, but changes the meaning. All factors considered, "Teacher Wang" may be a necessary evil, since the primary goal of this course is to teach Chinese. Now if this was the CH-EN course, I would strongly advocate setting the standard translation of 王老师 as Mr/Mrs. Wang and not accepting ""Teacher Wang", to make sure correct English is being taught instead of propagating misusage.
It should accept both "Mr., Mrs., etc." and "teacher." If no one says "teacher Wang" (and while you can never say never, generally speaking, no one every says "teacher Wang"), you really cannot expect them to translate it that way for the exercise, except after missing it once and hopefully remembering to say the incorrect translation the next time. It doesn't have to be one or the other. Both can be accepted. Otherwise, they can make a sentence like "Mrs. Wang is a teacher of [?] and only wants one cup of coffee." That way they can rest assured that the translator understands that "lao-shi" means "teacher," but also means "Mr./Ms. Wang" who is a teacher. The Portuguese course also does it this way.
I think "Teacher Wang just wants a cup of coffee" would be more natural/ common
Must 杯 be translated to "cup"? Because "mug" also makes sense, especially logically, only if the coffee is hot.