Sara está hablando sobre lo que necesita para la escuela. DICE: "Para escribir, necesito un lápiz".
Sara is talking about what she needs for school. She says she "needs something to write." So, the most logical ending would be "lápiz" because it is a tool for writing. With a "reloj" you can know the time and set alarms sometimes, but not write. The same goes for the "asignaturas", which are the subjects.
