Final answer:
The correct form of the verb is 'llegué' in the preterite tense.
Step-by-step explanation:
The correct form of the verb to fill the blank in the sentence is option A. llegué.
The original sentence in English translates to: 'When I arrived this morning, you had already left.'
The verb 'llegué' is in the preterite tense, indicating a completed action in the past.
Learn more about Spanish verb conjugation