The second answer. Because “sus” is the only one which has the “s” because it is plural. If it would be “nuestro” it should had an “s” like “nuestros hijos son altos” or “tus hijos son altos”
Comem(os), Jugam(os), Peliam(os) Comenzam(os). In other words pronouns that end in (os).
Yo trabajo en la oficina.
