The correct but unavailable answer is Rosario. Second best is at Rosario, based on the full answer of I go to school at Rosario. However, including a preposition with the answer is unneeded and clumsy.
Best to solve this one with some verbal algebra. Since the correct answer of Rosario is not offered we have to do some extrapolation to find which clumsy answer best fits the question. To make the question clumsy like the answer lets add an unneeded preposition to it.
Should my clumsy question be -
Where do you go to school to?
or should my clumsy question be -
Where do you go to school at?
The correct clumsy question is, Where do you go to school at, and the correct clumsy answer is at Rosario.