Final answer:
The correct word to complete the sentence is 'está'. 'Está' is the third person singular form of 'estar' indicating that someone or something is 'walking'.
Step-by-step explanation:
The correct word to complete the sentence is está.
When using the verb 'estar' to express an action or state of being in progress, we use the present participle form of the verb that ends in '-ando' or '-iendo'. In this case, 'está' is the third person singular form of 'estar' indicating that someone or something is 'walking' ('caminando').
Therefore, the correct sentence is 'Salvador está caminando al perro.'
Learn more about Verb conjugation in Spanish