Este sábado, Eric se [blank] como voluntario en algún lugar ecoturístico translates to "This Saturday, Eric will [blank] as a volunteer in some ecotourism place". The options are:
· reservaremos - the conjugation for nosotros - "we will reserve"
· reservaré - the conjugation for yo - "I will reserve"
· reservará - the conjugation for el/ella/usted - "he/she/you (formal) will reserve"
· reservarán - the conjugation for ellos/ellas - "they will reserve"
The subject of this sentence (Eric) is third-person singular, so the conjugation must be third-person singular. This means reservará is the correct conjugation.
Este sábado, Eric se reservará como voluntario en algún lugar ecoturístico.