Right answer:
b. ganamos
The word ganamos is given in the preterite tense while the word ganábamos is given in the imperfect. The preterite is used to describe actions that took place or were completed at a certain point in the past. On the other hand, the imperfect is used to talk about past actions, conditions, or events that occurred regularly or frequently or that were in progress in a moment in the past.
Since we won the match at a certain moment in the past, we need to use the preterite tense. That's why we choose ganamos that comes from the verb ganar and is the conjugation of this verb for the first person plural in the preterite tense. Finally:
El cinco de abril nosotros ganamos el partido finalmente