Answer:
The right answer is, D. Estudiaron.
* Ellos estudiaron para el examen esta mañana.
Step-by-step explanation:
Because when sentences are made in the preterite tense, these indicate actions that have been initiated in the past tense and that do not have a definite end, therefore the subject, gender and number must be identified within the sentence to be able to conjugate the verb correctly and that Be consistent with what is being expressed.