To me, public transport is correct not public vehicle. If the answer was vehicle, it should be in plural- Public vehicles.

The answer is 'public transport' and definitely not 'public vehicle'. Sometimes there are mistakes in the answer sections of some text books and that appears to be the case here. In British English, we don't use the collocation 'public vehicle(s)', it just sounds strange. Hope this helps!

A "public vehicle" specifically means a vehicle that people ride in to travel, with someone else driving - usually this means something like a taxi car or a bus. Unless it's pluralized, it needs an article like "a" or "the" in front of it to be grammatically correct ("The public vehicle was too small to be comfortable"). "Public transport" means any service people use to travel (except for airplanes, usually, which are considered their own category) where they pay to have someone else take them - this could mean a taxi, a bus, a train, a tram, or various other services. "Public transport" is what we would call a "blanket term," which is a term that is true of many things at the same time, while a "public vehicle" is a specific term that refers to the actual vehicle being driven. A taxi car is a public vehicle that is used for public transport - a car used for a public service.

Public transport is a system using vehicles. You can't say a system is expensive or cheap. Public vehicle I think is like bus or subway which you can estimate whether it's expensive or not.

Hello Trang, None of the answers for the question is right. It's supposed to be public transportation. Anything else is wrong. All of the answers don't even fit in the context of the question. I don' t know why the teacher thought so. The only reason I can give is that he is not a native speaker and doesn't know what is used in native speaking countries.