Answer:
We must complete the sentence:
"La ciudad —- muy bonita."
This can be translated to
"the city___ very pretty"
Now, "La ciudad" is a singular word, so the options C "eran" and D "fueron" can be discarded (because those are in plural)
Now, "Fue" and "Era" are almost synonyms, but "fue" is something that already happened, and "era" is used for an action that started in the past, but may still happen in the present.
So if we are talking about a city that is not today (for example any ancient city) the correct option is "fue"
If the city is one that still is today (for example, if you visited a city in the vacations, you can say "the city was very pretty")) in this case the correct option is "era"