Right answer:
b. creí
Creí is the conjugation of the verb creer for the first person singular in the preterite tense. The preterite tense is one of two simple past tenses. It is used to describe actions that took place or were completed at a certain point in the past. Also, this sentence is a negation because it uses the word no and this is used to contradict some or all of the meaning of a positive sentence. Finally:
Yo no creí lo que pasó. Todo fue de repente.
