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