Alright, so "fue" is ser in the preterite tense and "era" is ser in the imperfect tense, therefore there is no way "both A and B are correct".
Ser is the correct "to be" used since it describes a characteristic of the class. Therefore, "Neither A nor B are correct" is incorrect.
Next, we notice that there is no specific wording to show that the class took place in a specific time period, like last year or a few minutes ago. Therefore, its safe to assume that the sentence requires the IMPERFECT TENSE (era).
