Answer: Ella escuchó al perro aullar.
Explanation: Preterite form means that we are talking about something that happened in the past.
Nos, the sentence is:
She heard the dog howl.
The subject is "she", this means that the person in which we must conjugate is "ella" and the verb is "heard", the verb in Spanish is "escuchar"
Now, the sentence in Spanish will be:
Ella escuchó al perro aullar.