Final answer:
The more Gareth studies, the higher his test scores.
Step-by-step explanation:
The given question states that the amount of time Gareth spends studying and his test scores have a proportional relationship. By definition, a proportional relationship means that as one quantity increases, the other also increases or decreases by the same factor. C. The more Gareth studies, the higher his test scores is the correct option. This means that as Gareth increases the amount of time he spends studying, his test scores will also increase proportionally.