Final answer:
According to the statement, Charles is motivated by time off work.
Step-by-step explanation:
According to the statement, Charles is motivated by time off work (C). As mentioned, his valence will be highest if he receives an extra day of vacation for good performance. Offering Charles special sales tips (D) can help increase his expectancy, as additional training will help him do better at his job. Therefore, the factor that Charles is motivated by is time off work (C).