verbs - Which one is correct: "are (the) buses" vs "is (the) buses"?

This was a question from a test given to our students. The question was "What do you think the best form of transportation is for your city" Most of the students answer in this manner:

"I think the best form of transportation in my city is/are buses."

Which one is correct, "is buses" or "are buses"? Should there also be a definite article "the" before "buses"?

Answer

The verb should be is. There is no doubt about that, since the subject of the sentence The best form of transportation is singular.

However I do not consider the sentence idiomatic (perhaps because a singular subject and a plural object sounds awkward).

I would say:

The best form of transport in the city is (by) bus. The by is optional.

There are multiple examples on the site of choice of verb where the subject number differs from the object number - for example here