Correct Answer: D
Explanation:
A "bastion" refers to a fortress, but for most people not living in medieval France it more commonly speaks of a person or place that preserves or defends a more abstract notion. A woman who adheres to all the social rules of propriety and lives amongst neighbors who have no manners might be considered a "bastion of decorum." A radio station that alone plays harmonious music could be called a "bastion of euphony." "Mirage" would imply the mere appearance of learnedness (making it too similar to the surrounding lack of education), and "mirror" would require that the town itself be well-educated, too (so that there would be some local learnedness for the college to reflect).