The correct answer is A. fuiste.
The verb ser (to be) in Spanish is an irregular verb, which means that you need to learn its various forms by heart. In the example above, we need the preterite form of the verb ser, and it goes like this:
1. yo fui 1. nosotros fuimos
2. tu fuiste 2. vosotros fuisteis
3. el fue 3. ellos fueron
