Final answer:
The correct answer is 'di'. Marta les di las invitaciones a sus amigos ayer.
Step-by-step explanation:
The correct form of the verb to fill in the blank is di. The sentence should be Marta les di las invitaciones a sus amigos ayer. The verb di is the past tense form of dar, which means 'to give' in Spanish. In this case, Marta gave (di) the invitations to her friends.
Learn more about Spanish verb forms