Final answer:
The correct form of the present participle to complete the sentence is 'viviendo'.
Step-by-step explanation:
The correct form of the present participle to complete the sentence 'Sandra y yo estamos (vivir) en Tegucigalpa por un año' is viviendo.
The present participle in Spanish is formed by adding '-ando' to -ar verbs and '-iendo' to -er and -ir verbs. In this case, 'vivir' is an -ir verb, so the present participle is 'viviendo'.
Learn more about Spanish verb conjugation