Answer:
The options can be find by an online search, they are:
1.ser-yo
2.ser-tu
3.ser-el/ell/ud
4.ser-nosotros
5.ser-uds./ellos
6.ir-yo
7.ir-tu
8.ir-el/ell/ud
9.ir-nosotros
10.ir-uds./ellos
"Fui" is conjugated with "yo", the first person, and in singular, so the right answer will be the first option: yo, and "fui" means "I went", and is a conjugation of the verb "ir", so the correct option is 5: ir- yo