Right answer:
c. dijo
Dijo is the conjugation of the verb decir that matches the third person singular in the preterite tense. So Mi padre can be replaced by the subject pronoun for this person, which is él. 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. Finally:
Mi padre dijo que fue difícil
