The correct word in this sentence should be "vistieron."
Explanation:
1. The sentence is talking about "Ellos" which means "They" in Spanish.
2. The verb used in the sentence is in past tense, indicating that the action already happened.
3. The correct conjugation of the verb "vestir" (to dress) for "Ellos" (They) in past tense is "vistieron."
4. Therefore, the correct phrase should be "Ellos se vistieron rápido por la mañana."
So, the correct word or phrase to complete the sentence is:
- vistieron
