Answer: Ellos han llegado
Step-by-step explanation:
We will use the present perfect verb tense for this sentence. This involves the use of a conjugated verb of haber (to have) and a past participle. We can see with the "Hint" that the first two parts, they have, are already given to us as "ellos han."
They ➜ Ellos (subject pronoun)
have ➜ han (conjugation of haber in the present)
arrived ➜ llegado (past participle, ar to ado)
They have arrived ➜ Ellos han llegado.