Answer:
First, let's write a little translation of the text.
In many large cities such as Madrid and Barcelona, public transport is preferred for several reasons. What is a public transportation service used in large cities in Europe and Latin America, but not very common in the United States?
The ________ is a public transport used in large cities.
While in the US the subway is more used, in Europe where maybe the cities are a lot older, it is more common to see buses (or autobus in Spanish), which are really cheap and accessible to everyone.
The complete sentence would be
El autobus es un transporte público utilizado en las ciudades grandes.