Final answer:
The correct word is 'Es' and it is the third person singular form of the verb 'ser' (to be).
Step-by-step explanation:
The correct Spanish word to fill in the blank in the given sentence is 'Es'.
When answering the phone in Spanish, it is common to say 'Es' followed by the person's name. So, the complete sentence would be 'Es Marta allí?'
Here, 'Es' is the third person singular form of the verb 'ser' (to be), which is used to indicate identity or characteristic. In this case, it is used to ask if Marta is there.
Learn more about Using the correct word to answer a phone call in Spanish