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”
21. Yo paso tiempo con mi familia en Verano.
23. El chico no es ni trabajador ni estudioso.
24. Jorge debe levantar pesas en el gimnasio.
I believe that it would be B (reloj) since it is talking about how there is a window, a door and then a clock in a classroom.
It is also giving you the masculine singular form indicating that it can only be reloj.
