The question ¿Qué le pasó a Marlon? (What happened to Marlon in English) indicates that something happened in the past. So, the conjugation of the verb cortar in past tense is:
Yo corté
Tu cortaste
El/Ella cortó
Nosotros/nosotras cortamos
Ellos/Ellas cortaron
Marlon refers to el (he) and according to this, the correct form of the verb is cortó which means that the answer is Marlon se cortó el dedo.
